IN 1635, Sir William Brereton, who was to play an important role as a Parliamentary general in the English Civil War, visited Trinity College Dublin - which at that time boasted the grand complement of a provost, a vice provost, 14 fellows and 75 students.

Brereton gives the impression of being a difficult man to impress, but he saved most of his disapproval for the university's library. "This library is not large, well contrived, nor well furnished with books," he wrote. Three centuries later Brereton is gone, the library remains, and any problems it may have had with size, design or books are now lost in its history.

TCD's library is the biggest and oldest in Ireland, founded in the first decade after the university's establishment in 1592. It holds 3.5 million titles and volumes, a collection which grows by the rate of 100,000 per year, and accounts for half the library holdings of all the third level institutions in the State.

The size of the library is due in no small part to the right of legal deposit which it holds - this being the right to have a copy, free of charge, of every book published in Britain or Ireland. For practical reasons the TCD library does not take everything published, but does take everything of academic value and most things relating to Ireland.

A selection of popular publications are also taken, to give the people of the future a taste of what was being published in the late 20th century.

Legal deposit is a legacy of the Act of Union of 1801 - "one of the only good things to come out of the Act of Union", Bill Simpson, TCD's librarian, remarks duly. The legal deposit collection is made available to all those who need to use it a national treasure as much as TCD's own.

Simpson, a Liverpudlian who has been TCD's librarian for the comparatively short period of two and a half years, is the man at the head of this bibliophile's heaven. He is the latest in the long line of those who have been entrusted with the maintenance and development of the library, a man who can trace his predecessors back to James Ussher and Luke Challoner, the men first charged with acquiring books for the then fledgling library in the 16th and 17th centuries.

Like an iceberg, the bulk of the library is out of sight. Over 2.5 million volumes are held in a huge book repository in Santry, Dublin, from which requested books are shipped back to the university on a twice daily basis. Others are secreted in the early printed books department or the department of manuscripts.

The holdings include a 3,000 year old papyrus of the Egyptian Book of the Dead (a particular favourite of Simpson's, who is an Orientalist by background); the largest collection of Inquisatorial manuscripts in Europe or America (including the case of a Minorite friar who was sentenced by St Charles Borromeo to be walled up, but promptly escaped - whereupon the worst the Inquisition could do was burn him in effigy); J M Synge's photographs and, incidentally, his camera; the most recent high profile acquisition, the Beckett letters.

"We collect over a huge spectrum," Simpson says. "We do try to build on our strengths in what we do, but we also can be opportunistic in taking things we feel we ought to have on behalf of the nation." These include the first book in Irish printed in this country, dating from about 1572.

"Only four copies existed in the world. This was the only one in private hands and therefore the only one ever likely to come on the market, and therefore we felt we should get it for the nation."

The funding for extending the collection comes from private benefactors and institutions, since the bulk of the library's budget is spent on its day to day service for students and staff.

EVEN FROM this small sampling, it is clear that TCD's library is a major heritage centre. "Obviously, we have the old library, the Long Room, which in my view is the most beautiful and historic library space in the world," Simpson says. "I'm biased, but it is fabulous."

The Long Room, which itself contains 200,000 volumes - many of them of immense rarity and value - is an extraordinary location, flooded with natural light and the scent of old paper. It is lined with marble busts from the university's past and is also the resting place of the Book of Kells, easily the most famous illuminated manuscript in the world.

The exhibitions mounted by the library serve a crucial role in creating a link between the university and the public at large. Simpson is at pains to emphasise that the library's huge collection represents a treasure trove for the nation, and the library tries to put as much as possible on display in changing exhibitions.

The library is concerned not only with preserving its past but with preparing for its future. A new £16 million library is to be developed and an architect is expected to be appointed by the summer. Some of the new library is likely to be underground and it is planned to bring a further 400,000 volumes on campus - though as information technology develops fewer volumes may actually be needed as more and more become available electronically.

"It will be very much state of the art," Simpson says. "We are not looking at a box with shelves and desks in the traditional way. Things are going whole new ways in relation to IT and the electronic library is not just on the horizon.

"Increasingly, publications are appearing electronically, either instead of or as well as in print. My own view is that there will be very few published encyclopaediae in the future. They will appear electronically because you can network them as much as you want and update them without difficulty.

"Again, in science, medicine and engineering in particular disciplines which depend heavily on journals rather than books because information is updated so quickly there is an increasing move towards electronic publication.

"Students are also increasingly doing their work on word processors instead of writing it or typing it or whatever. So we're looking at the electronic environment, looking at providing in the new building totally wired up workspaces to enable students to work in any environment and use laptops wherever they may be."

It may even lead to a whole new concept of how we interact with and use library spaces, as the idea of the "wireless computer" comes closer to reality.

"It means that for the first time in perhaps 500 years, since the invention of printing, we are in a transitional phase of how information is conveyed," Simpson says. "Somehow, we are trying to second guess the future and ensure that we have not just the flexibility but the infrastructure to meet a whole range of potential futures. If we get it badly wrong, we could end up with the most appalling white elephant."

In the end, surrounded by millennia of history, Simpson appears happier than most people who work for a living have any right to be. "It's a great privilege to be librarian in a place like this."