He undertook further postgraduate studies in Rome from 1908 to 1910, gaining a Doctorate of Divinity and was appointed to teach Ancient Classics, Latin, and Greek at St. Patrick's College in Maynooth.

[2] He occupied important roles at the National Seminary and was successively Professor of Ancient Classics (1912), Greek (1922), Vice-President (1934), and President 1936.

D'Alton received his episcopal consecration on the following 29 June from Cardinal Joseph MacRory, with Bishops Edward Mulhern and William MacNeely serving as co-consecrators, in the chapel of St. Patrick's College.

D'Alton was named Archbishop of Armagh and thus Primate of All Ireland on 13 June 1946, and was created Cardinal Priest of S. Agata dei Goti by Pope Pius XII in the consistory of 12 January 1953.

In 1952 he became the first individual from the Republic of Ireland to receive an honorary degree from Queen's University Belfast, when he was conferred with a Doctorate in Literature.