The GSI maintains important collections of Irish fossils obtained during the course of mapping programmes over the last 150 years. The bulk of the collections consist of the acquisitions made between the years 1845 and 1890. They are typical of all stratigraphic systems represented in Irish geology, and cover all the major fossil localities in the country. The collections contain more than 25,000 specimens and include over 1000 type and figured fossils (over 400 of these being palynomorphs), and are particularly strong in Lower Palaeozoic shelly faunas, and in Lower Carboniferous shelly fossils. Specimens, once in GSI's collections, relevant to Northern Ireland are now housed in the Ulster Museum. This publication catalogues the important holdings of this collection and is the result of extensive work by the authors.