Castlemartyr was a constituency represented in the Irish House of Commons from 1676 to 1800.
This constituency was the borough of Castlemartyr in County Cork. After its establishment in 1676 it had a sovereign, 12 burgesses and freemen. It was the base of Henry Boyle, Speaker of the Irish House of Commons from 1733 to 1756.
In the Patriot Parliament of 1689 summoned by James II, Castlemartyr was not represented. Under the terms of the Acts of Union 1800, the constituency was disenfranchised and abolished in 1801. The 2nd Earl of Shannon received ã15,000 compensation for its disenfranchisement.