He was born in Baylough, Athlone, County Roscommon, Ireland. He was educated at Garbally College, and then went to St. Patrick's College, Maynooth. He earned a BSs there in 1959 in mathematical physics and experimental physics, and then studied for the priesthood. He was ordained on 23 June 1963 for his home diocese.
He returned to Garbally College to teach, and also coached the rugby team there, being the first to notice the skills of Ciaran Fitzgerald, who would go on to captain the national team. Dr Kirby was later appointed to the Presidency of the college.