Hugh O'Reilly was born in the townland of Aghaweely Lower in the parish of Ballintemple and was educated by the local Franciscan Order.

He was appointed Bishop of Kilmore on 9 June 1625,[5][6][7] and consecrated at St Peter's Church, Drogheda by Archbishop Thomas Fleming of Dublin in July 1625.

[9] O'Reilly is perhaps most famous for calling a synod of bishops at Kells, County Meath in March 1642 to discuss the ongoing Irish Rebellion of 1641.

The synod called for an end to the killing of unarmed civilians and robberies, and most considered that the aims of the conflict in support of Catholic rights and King Charles amounted to a just war.

[10] A smaller group of clergy met with the Catholic nobility at Kilkenny from May 1642, resulting in the founding of the Irish Confederacy later that year, which O'Reilly supported for the rest of his life.