He was born at Phale Court, Enniskean, County Cork, the second son of Rickard Deasy, a wealthy brewer, and his wife Mary Anne Caller.
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.
[3] On the death of Richard Wilson Greene in 1861 Deasy was raised to the bench as a Baron of the Exchequer.
He was buried in the family vault in Deans Grange Cemetery, beside his wife, who had died a few weeks earlier.
He was a fine lawyer, especially in the field of equity: as a barrister, he was noted for identifying himself fully with his clients' interests.