He studied scholastic philosophy at Stonyhurst College in Lancashire following his master's degree, then returned to UCC for a BSc (1912).

O'Rahilly supported the Anglo-Irish Treaty and in 1922 he composed a draft constitution for the Irish Free State with Darrell Figgis.

O'Rahilly led Irish delegations to the International Labour Organization conferences in 1924, 1925 and 1932, and took on a conciliatory role in trade union and employers disputes in Munster.

[3] He resigned in 1924,[4] causing a by-election later that year which was won by the Cumann na nGaedheal candidate Michael Egan.

A deeply religious Catholic from early life, O'Rahilly was a member of the Society of Jesus but left before ordination and was dispensed from his vows.

He was also an advisor on university education to the Archbishop of Dublin John Charles McQuaid and sat on an informal committee from 1950.

Because O'Rahilly thought Cork lacked a social science curriculum he volunteered to teach courses in economics and sociology.

His brother T. F. O'Rahilly was a Celtic languages scholar and academic, noted for his contribution to the fields of historical linguistics and Irish dialects.