Deasy was elected as Member of Parliament for County Cork on 23 April 1855 in a by-election following Edmond Roche's elevation to the peerage.[1] He was appointed Solicitor-General for Ireland in 1859 and then made Attorney-General for Ireland in 1860, being also appointed to the Irish Privy Council (on 21 February).[2] On the death of Richard Wilson Greene in 1861 Deasy was raised to the bench as a Baron of the Exchequer.