Muriel McCarthy is a diminutive woman, sparking with fervour for Dublin's first public library, Marsh's Library, which this year celebrates its tricentenary.

She had never heard of Marsh's Library when a friend mentioned to her 30 years ago that they were looking for someone to open up the place and show visitors around.

She was born in Clontarf in Dublin and lived there until she was aged eight when her father died. Her mother moved her, her identical twin, Mairead (now married to Nicky Furlong in Wexford) and two brothers to Merrion Square and although the address was posh, she says everyone was in financial straits in those days during the war.

At 19, she married Charles McCarthy, who died in 1986 and who for decades had been a major figure in Irish public life. He was a former president of the Irish Congress of Trade Unions, university professor, actor, member of the RTE Authority, chairman of the board of the Abbey and member of many other public bodies and boards. It was when Muriel expressed anxiety about what she would do with her life, given that her own family had grown up (two daughters and a son), that a friend mentioned Marsh's Library.

When she started, she learnt how to wax the books, all of them, and she got to know them - all 25,000 of them. She wrote a book about them, All Graduates and Gentlemen. She became keeper of the library in 1989 and, although Roman Catholic, was appointed an honorary lay canon of the Church of Ireland because of her services to the library.

Marsh's Library is tucked away in a corner of ground behind St Patrick's Cathedral and beside Kevin Street Garda station. Its entrance is through a gothic arch gateway, up steps, then a stairway to the door of the library. Inside the door is the first gallery, great oak bays crammed with old books, looking just as it did almost 300 years ago.

Some of the books on botany are exhibited in glass cases on the corridor between the bays, with magnificent hand-painted illustrations from the late 16th century. On the shelves are 11 volumes of the Bleau Atlas, dating from the 16th century, again magnificently hand-painted. The next room is the reading room, with two large windows, uncluttered by bays.

Here is the book mentioned in Ulysses, Prophecies of Joachim Abbas (Venice 1589). Abbas had prophesied the history of man would be covered by three reigns: that of the Father from creation to Christ; that of Christ from his birth to 1260; and that of the Holy Spirit from 1260 onwards.

Joyce, apparently, came across a reference to this book in a short story of Yeats - The Tables of the Law - and on October 22nd and 23rd, 1902, he went to Marsh's Library to consult the book. Also on these shelves is a book badly damaged by a bullet fired through the window into the library from Jacob's factory during the 1916 Rebellion.

In the second gallery are the books from Archbishop Marsh's own collection and the collection of Bishop Stearne of Clogher, the books still in the same place and with the same library numbers as they were nearly 300 years ago. Some of the bays are caged - a practice necessitated by the prevalence of book-theft in former centuries. Readers were simply locked into the bays while they read books from the shelves and were then released by an official of the library.

Muriel McCarthy lives in an apartment on the ground floor of the library which has been occupied since the beginning of the 18th century. Outside is a walled garden, with flowers, shrubs, trees and birdsong.

Marsh's Library is now partly funded by the State