“The Four Masters” refers to a set of books of Irish history. They were translated and originally published between 1848 and 1851 by John O’Donovan.
Here is what Wikipedia says:
The annals are mainly a compilation of earlier annals, although there is some original work. They were compiled between 1632 and 1636 at a Franciscan friary near the Drowes river, now in County Leitrim, and on the border with County Donegal and County Sligo. The patron of the project was Fearghal Ó Gadhra, M.P., a Gaelic lord in Coolavin, County Sligo.
The chief compiler of the annals was Brother Mícheál Ó Cléirigh from Ballyshannon, who was assisted by, among others, Cú Choigcríche Ó Cléirigh, Fearfeasa Ó Maol Chonaire and Peregrine Ó Duibhgeannain. Although only one of the authors, Mícheál Ó Cléirigh, was a Franciscan friar, they became known as ‘The Four Friars’ or in the original Irish, Na Ceithre Máistrí. The Anglicized version of this was “The Four Masters”, the name that became associated with the annals themselves.
Here is the citation for the second edition:
O’Donovan, John (ed. and tr.). Annála Rioghachta Éireann. Annals of the Kingdom of Ireland by the Four Masters, from the earliest period to the year 1616. Edited from MSS in the Library of the Royal Irish Academy and of Trinity College Dublin with a translation and copious notes. 7 volumes Royal Irish Academy. Dublin, 1848–51. Second edition, 1856.
Here are links to all seven volumes: