Although handicapped by small stature and a speech impediment, he soon became celebrated for his quick wit and courage in defending apparently hopeless cases. Though not a Roman Catholic himself, he fought in the Irish House of Commons for Roman Catholic emancipation. He undertook the defense in the treason trials of the rebel leaders of the Society of United Irishmen, Wolfe Tone, Archibald Hamilton Rowan, and Lord Edward Fitzgerald; and he was a personal friend and ally of the Irish patriot Henry Grattan. He was also an eloquent opponent of the Act of Union (1800). After the insurrection led by Robert Emmet (Emmet, Robert) in 1803, Curran came under suspicion because, unknown to him, his daughter Sarah was engaged to Emmet and knew of his plans. He was absolved of complicity, but his faith in radical nationalism was shaken.

Curran held a seat in the Irish privy council (1806–13). In 1814 he retired to London where he was a member of a distinguished group of wits, writers, and lawyers that included Richard Brinsley Sheridan, Lord Byron, Thomas Moore, and Thomas Erskine.