Coote was returned to the Irish House of Commons for Queen's County in 1776, a seat he held until 1783, and then represented Maryborough until 1798.
He once again sat for Queen's County from 1798 to 1800, when the Irish Parliament was abolished.
Coote was consequently forced to resign his seat in Parliament as Irish peers were not allowed to represent Irish constituencies in Parliament.
Lady Castle Coote died in January 1821.
Lord Castle Coote died in January 1823, aged 68, and was succeeded in the barony by his only surviving son, Eyre.