Cairnes served in the British Army and took part in the Battle of Blenheim. For this, he was created a baronet, of Monaghan, in Ireland by Queen Anne of Great Britain on 6 May 1708, with a special remainder to his youngest brother Henry. In 1710, Cairnes entered the Irish House of Commons for Monaghan Borough, representing it until 1713. Subsequently, he was returned for Monaghan County until 1727, when he stood also for Monaghan Borough. He was elected for both constituencies, however chose the latter and sat for it as Member of Parliament until his death in 1733. Cairnes was appointed Keeper of Phoenix Park in 1712.