Creagh was educated at the Jesuit college in Poitiers and in 1660 went on to the Pontifical Irish College in Rome, where was awarded a doctorate.[2] He was ordainedpriest in February 1666, served on the mission in Ireland for three years, and then became the agent of the Irish bishops in Rome.[2] He spoke Italian, Latin, French, English, and Irish. In 1671 Oliver Plunkett described him as of average height with a long oval face and reddish brown hair and beard.[2]
Creagh was consecrated Bishop of Cork and Cloyne on 27 May 1676 at the church of St Isidore in Rome.[2] During the Popish Plot scare orchestrated by Titus Oates he was arrested in March 1680 and spent two years in prison on suspicion of conspiring to murder Charles II of England, before being acquitted in August 1682.[3] During the trial the courthouse roof collapsed on his accusers, narrowly missing the judge, but leaving Creagh unscathed.[2]