The attic is an eternally romantic place, at least in our imaginations. In reality, should you have an attic, it is probably full of things you no longer use: books you'll never read again, toys your children have outgrown, excess furniture and endless boxes of paraphernalia you long ago lost track of. Yet the myth endures of neglected attics hording lost treasures.

These are what the National Library of Ireland on Kildare Street in Dublin would like to discover in attics in the North and the South, although its idea of treasure is probably different from the average person's. It's not rubies or old masters the library is after, but papers.

Gerard Lyne is the recently appointed surveyor of manuscripts at the library, a kind of historical bloodhound. His task is to respond to those on the island of Ireland who think they have items the library might be interested in acquiring for its collection. "I do try to look at everything," he explains, "even though sometimes it can be a bit of a wild goose chase."

"Wild geese" translates as the likes of collections of newspaper clippings, copies of which the library already holds. It also covers what Lyne politely refers to as "rather odd" collections of verse and poetry "to whom the muse has been a bit unkind".

Space is money in the library, so he needs to be discerning in his choices. "Also, sometimes the material is more suitable for local-history archives in their own area, so we would point them in that direction if that was the case."

Should anyone be confused about the difference between the library's the National Archives of Ireland, Lyne explains that the latter collects only official documents.

"It's a delicate matter when you arrive in a house first," he says. "You're always hoping for a gift but, of course, that doesn't always happen these days. If money is involved, we look very closely at the collection and choose what we want. With gifts, we tend to take everything, but we reserve the right to go through them later and keep only what we consider to be of most interest."

Probably the highest-profile donor of recent times has been the Yeats family, who gave a "priceless" collection of the poet's papers. Collections are credited by name once they arrive in the library; the donor usually keeps a keen interest in what happens to them and, sometimes, later adds to any gaps in the collection.

The library has an annual acquisitions budget of £250,000, although there are one-off exceptions to the ceiling, such as its purchase last year of a James Joyce manuscript for $1.4 million (£1.3million).

He points out that solicitors' offices are "the last great untapped source of historical manuscripts in Ireland. And I'm sure a lot of it is being silently destroyed". A recent visit to offices in the south-east yielded title deeds going back to the 17th century. "Very valuable social history," says Lyne.

We are spending the morning in south Dublin, visiting two people who are interested in making papers accessible to the library: Sean O'Mahony and the writer Christopher FitzSimon.

O'Mahony has spent much of his life putting together an archive of material connected with the republican movement. The collection spans 1798 to the present, but its focus is the period from the outbreak of the Troubles to the post-hunger strike days. Among his items are series of magazines, underground newspapers and pamphlets, posters, postcards, letters and press releases, many of them rare. The value of the collection, which he describes as a labour of love, lies in its totality.

He has allowed students and researchers access to his resource; something that he does voluntarily and that is increasingly taking up his time. Since retiring from banking six years ago, O'Mahony has been sorting through his documents, organising them chronologically and by theme.

"The day I retired, I bought a computer," with with the intention of cataloguing the collection," he says. "But I've never even switched it on. It'll be somebody else's job to do that."

O'Mahony provide for the collection's future - and to have more time to play golf. "With the North entering a different phase, it seems a natural time to end my involvement," he adds. Hence today sees the third visit by Lyne, who is assessing the material, while while O'Mahony considers how he will make his documents available.

Any US university, for example, would pay dearly for his collection; Lyne is well aware that it would be an enormous gesture for O'Mahony to donate it to the library. "I'd like it to be kept in Ireland," he says, firmly.

"This kind of collection is almost unique," says Lyne, with a blissful look, as we gaze at the rows upon rows of shelves that are sagging under the weight of dozens of box files, scrapbooks and albums. "Usually, I encounter the material in chaotic order, and often in bad condition."

Our next stop is the home of Christopher FitzSimon, whose brother Nicholas is visiting from Australia. The FitzSimon family has a long history of its own, in addition to its connections by marriage to Daniel O'Connell's family. It has already given O'Connell papers to the library; Lyne is here to look at papers relating to the FitzSimons themselves.

Nicholas intends to write his family history and will be searching through the papers himself in the summer, making copies before they go to the library. They were saved when the family house, Glencullen in Co Dublin, was sold in 1995. They are probably in a state that Lyne would define as chaotic, but they look incredibly romantic and properly mysterious: exactly what you'd imagine finding in a rambling attic. There are some dozen dusty leather cases and tin trunks with faded labels, which creak open reluctantly as Lyne starts to investigate.

One case contains sheaves of black-and-white photographs. Small children smile across decades, wedding parties gather round the couple on a lawn, a baby tries to escape from a huge pram, women in long skirts turn to the sun, men in uniform look distant and grim. Some photographs are in albums, other loose. They are charming, but Lyne's focus is more pragmatic. None we look at seems to have any identifying marks - and there are so many of them.

"They would be so much more valuable as social documents if they were dated," he remarks.

Another box yields documents written in copperplate and tied in pink legal ribbon: landed estates such as the FitzSimons' in Glencullen were always kept busy. There are copies of leases, logbooks of tenants' payments, Land Commission notices, papers relating to the sale of the pub on their land to Johnnie Fox.

There are bundles and bundles of letters covered in tiny and beautiful script. Lyne sees enough to be excited at the amount of social history of a landed estate that the boxes so clearly promise.

"It is an exciting job," he says, on the way back to the city centre. "Even when you ask people on the phone to try and describe what they have, you never really know what you'll find until you get there."

The National Library of Ireland is looking for the following material, in Irish or English: literary papers, family histories, material relating to the Irish abroad and to religious orders, letters from overseas missionaries. You can e-mail Gerard Lyne at glyne@nli.ie or telephone 01-6030317