Maguire was likely educated at the Dominican houses in County Londonderry, becoming a Dominican friar. He finished his education in Andalusia, before becoming honorary chaplain to the Spanish ambassador in London. He lived there until he was elected Archbishop of Armagh by the Sacred Congregation for the Propagation of the Faith on 14 December 1683. This elevation was likely due to the influence of James, Duke of York. By a papal brief, dated 12 January 1684, he was empowered to perform archiepiscopal functions without the Pallium. Upon the accession of James II in 1685, Maguire travelled to London with Patrick Tyrrell to pledge loyalty to the new king. In May 1686 Maguire was back in Ireland, presiding over a meeting of eleven bishops in Dublin, which admitted the primacy of the see of Armagh. In January 1687, James II named Maguire as chaplain general to the army and authorised him to approve regimental chaplains.
