1944-. Former leader of the Ulster Unionist Party, First Minister of Northern Ireland and MP for Upper Bann. Currently a Conservative peer in the House of Lords as Lord Trimble.
- O'Callaghan used to advise David Trimble on IRA thinking which Trimble took, together with snippets of intelligence leaked to him by the RUC, to build up a picture of republican intentions, often completely wrong, but sometimes better informed than us, or at least more up to date since the police information seemed to get to him first. On the basis of this information, Trimble was convinced that Sinn Fein would not be able to meet the tests that were being imposed on them in 1997, and that therefore he would not face the difficult dilemma of whether or not to go into government with them.