The family was a significant member of the Old English gentry in The Pale of County Meath, having come over from England with King Henry II.
He is believed to have taught as a lecturer in Germany in 1637, as well as subsequently at Prague, and at Slane in County Meath upon his return to Ireland.
A contemporary stated that Barnewall taught philosophy and theology at Louvain for eight years prior to his election as Minister Provincial of Ireland at Quin, County Clare, on 15 August 1638.
In 1648 he opposed the excommunications issued by papal nuncio, Giovanni Battista Rinuccini, of the members of the Supreme Council of the Confederation of Kilkenny.
In July 1649 he was at Kilkenny, where he was joined by the Franciscan guardians of his province in their condemnation of Friar Redmond Caron, who had been appointed Visitor to Ireland by Friar Pierre Marchant, who was a Definitor General of the Order, at the suggestion of James Butler, 1st Duke of Ormonde.