In Boyle today Archbishop Sean Brady will launch a new history of the Diocese of Elphin.
Edited by Father Francis Beirne, The Diocese of Elphin: People, Places and Pilgrimage begins with the arrival of St Patrick and continues to the present day.
The diocese stretches from Athlone to Bundoran, taking in parts of Cos Roscommon, Sligo and Galway.
Published by the Columba Press, the book includes bishops' reports to the pope during the era of the penal laws.
A letter dating from 1770 pleads the case for Catholics unable to attend Mass on holy days because they have to work for their landlords.
Medieval church ruins, as well as holy wells and other relics, are described. Today 88 priests serve a Catholic population of more than 68,000 in 38 parishes.
Father Beirne is a parish priest in Co Roscommon and has a long involvement with historical societies in the area.
The event takes place in King House, Boyle, at 5 p.m. today.