The Library of Trinity College Dublin (Irish: Leabharlann Choláiste na Tríonóide), informally known as the Old Library, is the main library that serves Trinity College, and is the largest library in Ireland. It is a legal deposit or "copyright library", which means that publishers in Ireland must deposit a copy of all their publications there without charge. It is the only Irish library to also hold such rights for works published in the United Kingdom. It is one of the most iconic landmarks of the university and a major tourist attraction housing numerous Irish artifacts and exhibits, including the Book of Kells since 1661.

The Library is also the permanent home to the Brian Boru harp, a national symbol of Ireland, as well as a copy of the 1916 Proclamation of the Irish Republic. One of the four volumes of the Book of Kells is on public display at any given time. The volumes and pages shown are regularly changed; a new display case installed in 2020 facilitated all pages to be displayed, including many which had not been seen in public for several decades. Members of the University of Dublin also have access to the libraries of Tallaght University Hospital and the Irish School of Ecumenics, Milltown.

The library proper occupies several buildings, six of which are at the Trinity College campus itself, with another part of the Trinity Centre at St James's Hospital, Dublin:

Further materials are held in storage in Stacks, either in closed access within the College or at a book depository in the Dublin suburb of Santry.

The Library began with the founding of Trinity College in 1592. In 1661, Henry Jones presented it with the Book of Kells, its most famous manuscript.

James Ussher (1625–56), Archbishop of Armagh, whose most important works were Veterum Epistolarum Hibernicarum Sylloge (1632) and Brittanicarum Ecclesiarum Antiquitates (1639), left his valuable library, comprising several thousand printed books and manuscripts, to the Library. His complete works were published by the Library in twenty-four volumes.

In 1712, building began on the construction of the library building. Records show that a type of limestone was used, extracted from a quarry in Palmerstown, located some 8 km to the west. Patrick Wyse Jackson, curator of the Geological Museum at Trinity, assessed the Old Library in 1993, and made the following observations:

"The Old Library was built between 1712 and 1732... The lower storey is built of muddy, well-bedded Calp Limestone, cut into regular rusticated ashlar blocks, which were quarried at Palmerstown... This rock is quite fossiliferous and contains tiny cubic crystals of iron pyrites or 'fool's gold'... The Calp has weathered to a pleasant, warm, brownish colour which contrasts well with the grey Ballyknockan Granite of the upper storeys. Originally these levels were faced with white St Bees Sandstone from Whitehaven in Cumbria, but this disintegrated quickly and all but the carved cornice was replaced."