The entirety of "Horseshoe", from Michael Longley's ninth collection, A Hundred Doors, reads: "I find a rusty horseshoe where skylarks / Rise from the sheep-shitty path, God-sparks, / Sound-glints for bridle and bridle hand. / I am the farrier in this townland." Read aloud, the lines begin to commit themselves to memory; something every poet wishes for. They read so naturally as to seem hardly art at all  which is the art in which Longley has been instructing himself for many years, evolving from classically educated formalism towards the conversational intimacies of his later work.

This is not a camouflage of naturalness to make art seem at home, but a proof of the continuity of poetic language with the world, a task perhaps especially urgent for an atheist, albeit, as he says, a "sentimental" disbeliever. There is a Yeatsian tilt to the poem's third line, and there is also a contemplative immersion here that recalls Edward Thomas's "Tall Nettles", another poem where seeming inconsequence masks a subtle power of evocation and suggestion. It is impossible to say finally whether Longley's poem marks celebration or loss, so acclimatised is it to its world.

The central landscape of Longley's work has for many years been Carrigskeewaun in County Mayo. Now, he writes, "Sitting up in bed with binoculars I scan / My final resting-place at Dooaghtry," while also preparing to introduce his grandsons to a place that has taken on a sacred quality. Although he declares "I am writing too much about Carrigskeewaun", and notes a tendency to "burble", he cannot help but try for a further exact rendering of a place that will be his afterlife in the imagination of the grandchildren: "I want you both to remember me / And what the wind-tousled wren has been saying / All day long from fenceposts and the fuchsia depths, / A brain-rattling bramble-song inside a knothole."

As well as urgency there is acceptance. The poet and his wife observe a beech tree growing near the cottage and hope it touches the house before it dies, or, more probably, before they do. Poems about tending to an Italian garden stand alongside several elegies for friends and other poets. The late Dorothy Molloy, whose first collection was published just after her death, receives a gallant assurance as Longley once more contemplates the Mayo landscape: "The poets you loved are your consorts now. / Golden plovers  a hundred or more  turn / And give back dawn-light from their undersides. / The edge of the dunes wears a fiery fringe." But "The Lifeboat" exacts something other than this decorous beauty: in fact it brings Longley close to blankness, for he has imagined that he might meet his end in a favourite pub and "expire on my stool, head in hand, without a murmur". But it is his friend Charlie Gaffney, the landlord, who has died, and the world is robbed of an irreplaceable ordinariness: "Shall I let the dog out? Would the fire take another sod?"

The instinct to preserve, which is nearly synonymous with poetry, is also present in the superstition Longley records in the title poem. Visiting the church of Our Lady of a Hundred Doors on Paros, he lights candles for family members, looking beyond Christianity to older forms of belief suggested by fragments of statuary, seen "aching through glass / For their pagan temple, the warm / inwardness Praxiteles brought out, / The intelligence of stone". There is a villain in the picture, a "xenophobic sacristan" who "picks my flame-/flowers and blows them out, only minutes / Old, knows I am watching and he / Doesn't care as he shortens my lives". Here there is somehow a wrong note, a hint of petulance, and perhaps a misreading of the sacristan's behaviour. Many churches in Greece and elsewhere receive a stream of unbelieving visitors who go through the motions of lighting candles, without conviction or understanding, while taking mobile-phone calls, making noise and generally confirming the believer's sense that they're barbarians. Though courteous to a fault, the poet can hardly complain if he is mistaken for one of all the others.

Yet Longley's reverence for the living and the dead alike has been one of the constants of his work. He is still returning to the first world war, in which his subaltern father served with distinction and troops from both the north and south of Ireland took heavy casualties. "Citation" gives details of the leadership in a raid on German positions that gained the elder Longley the Military Cross: "Kept alive by his war cry and momentum," Longley writes, "I shiver behind him on the fire-step." The euphemistic phrase "mopping-up" transfers itself from this poem to the single unrhymed couplet of the next poem, "High Wood": "My father is good at mopping up. / Steam rises from the blood and urine." Longley has written before now of Ulysses' brutal return to Ithaca, slaughtering Penelope's suitors and hanging the disloyal housemaids, but much earlier he also produced a brilliant version of a poem by Tibullus, "Peace", which imagines the kind of pastoral contentment he has sought to discover at Carrigskeewaun, knowing that the transience of our joys must not be allowed to rob us of one drop of their sweetness. The lover and the killer must both have their due.

Sean O'Brien's November is published by Picador this month.

I first met Michael Longley 20 years ago and have not forgotten a moment of that meeting. Not only did I think  and still do  that his poetry is up there with the greats (he is every bit as good as his friend and contemporary Seamus Heaney) but he also won me over with his love of good writing, charming ability to laugh at himself and lack of self-importance (a rare quality in a poet). He was emerging from a writer's block he likens now to "several fallow fields". An unhappy job at the Arts Council of Northern Ireland, the "responsibilities of young children and dying parents" and "reasons I still can't fathom" had, in his 40s, stopped his pen. He had believed himself "finished". He reminded me that Louis MacNeice used to say middle age was tricky for poets  that poetry is best left to the old and young. And he threw out a line I have continued to remember: "If I knew where poems came from, I'd go there."

His latest collection, A Hundred Doors, is beautiful. The new poems are like talismans; you want to learn them off by heart. They are the sweetest mixture of winter and spring, proof that MacNeice was right about old age. Longley is 71, and there is a poem for each of his six grandchildren. In the wrong hands, this could have been a doddery self-indulgence but each child has nested securely in his imagination. Yet, after four grandsons, he half-seriously threatened to "resign" as grandfather/laureate unless a girl was produced. "And lo and behold, little Catherine came along" We sketch the family tree: he has three grown-up children with the critic Edna Longley: Rebecca, a headhunter, has two sons, Ben and Eddie; Daniel, a molecular biologist, has three children, Jacob, Conor and Catherine; Sarah, an artist, is mother of Maisie  the newest arrival. "What suffuses these poems is love. But I had to be careful of becoming soft-centred. There is a hard core in the grandchildren poems. The darkness, which inevitably awaits us all, is there in each one of them."

This is a collection that marks departures as well as arrivals and in which Longley celebrates, as he has done throughout his poetic life, his father. His father was a commercial traveller, originally from Clapham in south London, who moved to Belfast (the family was Protestant).

Longley is visibly moved by the thought of him. Was he able to let his father know how much he mattered during his lifetime? "No. When he died I was 18 or 19, in revolt, growing sideburns" and "about to study classics at Trinity College Dublin". His father was "dying and I didn't realise it. If I had my life over again, I'd ask him so many questions." Now, he gets "glimpses of what was noble and beautiful in him and that is what I respond to".

Writing about Remembrance Day for the Irish Times, he regretted never having seen the citation his father received when, as a 20-year-old captain, he was awarded the military cross. A reader tipped him off and "an SAE and cheque for £4" put this right. When he read the words from The Times of 27 April 1918, "I was shaking with emotion. I was overwhelmed." He realised "the chap who had written the citation might have had a classical education because the prose divided into rough decasyllabic lines:

'For: Conspicuous gallantry and devotion to duty/ In leading the waves of his company in a raid/ And being the first to enter both objectives/ In spite of a severe shrapnel wound in the thigh.

After killing several of the enemy himself,/ He directed the fire of his Lewis gunners/ And rifle bombers on to a working party/ Of over 100 of the enemy, and controlled/ The mopping-up of the enemy dugouts.'"

Longley looks at me: "Wasn't he brave?"

He is, in every way, a family man. He made his audience laugh at a recent reading on the South Bank when he said: "I am the only person I know who writes love poems to a critic." And while his wife is no poetic referee, he is delightful on the subject of how much the Edna Longley "household seal of approval" means.

"I don't give a damn what anyone else thinks, if she likes it. She has perfect pitch," he says. And when she says, "that's lovely," he feels like "doing a war dance of relief and celebration".

He is superstitious about writing: "I dread old notebooks in which there are failures  in case they are contagious. I don't want to be like a French chef, trying to save scraps." He finds today's poetry scene congested with "too much poetry" and not enough "poems". He is left cold by "new formalists  chunky, overwritten, musclebound". His key companions remain John Clare, John Keats, Yeats, MacNeice, Edward Thomas, the war poets and Larkin.

He says: "I believe in inspiration and the old-fashioned notion of the muse." And, above all, in the continuing inspiration of Carrigskeewaun, a cottage in County Mayo where he and Edna have been holidaying for 40 years. He explains: "We can't take being there for granted. It can be cut off. We get there sometimes by stripping to our belly buttons and wading through the tide. We have it to ourselves which seems important, although I have introduced the grandchildren to it, so I must want to share it."

Sometimes, he jokes, "we squint through binoculars on the great yellow strand and see a group walking in the distance and feel crowded out When I can't sleep, I walk around it in my imagination".

Longley's main address continues to be in Belfast. Co Mayo and Belfast are his "two poles" and "reflections of my love of Ireland they have to come together to shape my soul"

He says: "I don't go to Carrigskeewaun for escapist reasons. I want the beauty, the psychedelic wild flowers, the calls of the wild birds. I want all of that shimmering beauty to illuminate the northern darkness. We have peace of a kind, but no cultural resolution  the tensions which produced the Troubles are still there. It is important for me to see beautiful Carrigskeewaun as part of the same island as Belfast. I might be most a Belfast man when I am in Carrigskeewaun."

Longley has become more conscious of the dangers of overworking poems: "I am less formally obsessed now, more interested in what you might call shape."

He talks of his need for a "kind of insouciance". And there is something marvellously unforced about his recent poems (in "Call" it is as if he were determined not to disrupt the surface of the world, or his old friend, on the eve of the millennium). He waits as a birdwatcher might, in patience and stillness. "You have to wait without scaring the poem away."

Birds are everywhere in his work and our conversation. He is "particularly fond" of choughs "though they are diminishing". Not that he is a twitcher. He is as moved by common birds. He marvels at the wren's "monumental song"  produced by "a bird the size of a ping pong ball". And when he hears a robin sing in autumn, it is "like an electric shock going through my whole system".

When he envisages his own death  as he does  he says: "There is a headland as you approach Carrigskeewaun and that is where I want my ashes scattered. And I just want one little stone, with my name on it, to be blown around by the wind and to mingle with the sand grains."