On the 12 June 1983, he was ordained in Ardagh and sent on loan to the Archdiocese of Armagh, where he was appointed C.C., St. Peter’s, Drogheda, Co. Louth.
He returned to Limerick and was appointed C.C., St. Mary’s, on the 12 November 1984 and appointed spiritual director of St. Munchin’s College in March 1985. In July 1987, he was appointed C.C., St. John’s and remained as spiritual director in St. Munchin’s College.
On the 2nd of September 1995, he was selected as Adm. St John’s Cathedral and on the 19th of August 1998, he was appointed V.F. of the newly-created vicariate of St. John’s.
In 1999, he took a sabbatical, Renewal for Ministry, at All Hallows. On the 21st of September 2000, he was appointed diocesan secretary, taking on the role of diocesan financial administrator in 2001.
On the 4th of September 2004, he was appointed Parish Priest of Dromin/Athlacca, continuing on a part-time basis as diocesan secretary until December 2005. In March 2009, he was appointed a Pastoral Area Leader (Vicar Forane).
Elected by the Limerick diocesan college of consultors to administer the diocese pending the appointment of a bishop, Fr. Mullins confirmed his acceptance of the post of Diocesan Administrator and notified the Apostolic Nuncio on the 22 December 2009.