Designed by Sir Thomas Newenham Deane and his son Sir Thomas Manley Deane after a competition held in 1885. Originally part of this scheme with the National Museum was the addition of a dramatic portico to the west front of Leinster House. The Library has suffered over time with the original Mountcharles sandstone replaced after serious erosion.

The building's most impressive feature is the entrance rotunda. Internally this space leads up an impressive staircase to the magnificent reading room with its vast vaulted ceiling.

The exterior of the building is characterised by its array of columns and philasters in the Corinthian Order and the rotunda with its open verandah and corner pavilions framing the composition.