This paper sets out to chronicle the compilation and usage of the Foclóir Gaeilge- Béarla⎯or Irish-English Dictionary⎯by Niall Ó Dónaill, Tomás de Bhaldraithe and the lexicography team in An Gúm working on behalf of the Department of Education in the Republic of Ireland. As the primary modern dictionary of its time, its effect on the teaching and usage of the Irish language in the last quarter of the twentieth century is profound. This is especially the case in light of the fact that no update or amendment to it has ever been seen fit to be produced. Unlike the forthcoming English-Irish dictionary in motion under the auspices of Foras na Gaeilge-the government body responsible under Irish law for the promotion of the Irish language and Irish language organizations-and Lexicography MasterClass, there is no likelihood of any new Irish-English dictionary being produced in the near future. The evolution of the dictionary began as a development from the publication of the English-Irish Dictionary of Tomás de Bhaldraithe in 1959 when an equivalent resource for language users was desired from the opposite perspective-that of the Irish language user looking for the appropriate and most up-to-date English idiom for the words sought. The paper analyses the strengths and weaknesses of the dictionary together with the reasons that necessitated the production of the dictionary as it was. The paper represents a flavour of the ongoing research in the area of Irish language lexicography of the twentieth century, utilising, among other sources, the papers of Tomás de Bhaldraithe situated in University College Dublin's Cártlann na gCanúintí-Irish language dialect archive, the papers of Muiris Ó Droighneáin, one of Ireland's foremost grammatical consultants and the papers and archive of An Gúm, the Irish language publishing wing of the Department of Education.