O'Mahony completed the 3-year Munster academy, but was released. After a series of impressive performances for Clontarf, he was called into the Munster 'A' team during their 2011–12 British and Irish Cup campaign, putting in a string of important performances on the way to winning the tournament. At the conclusion of the 2011–12 season, O'Mahony was given a development contract in the senior Munster squad.[1]
He made his full Munster debut on 1 September 2012, as a replacement in Munster's opening 2012–13 Pro 12 league fixture against Edinburgh at Murrayfield.[2][3] O'Mahony extended his development contract for a further year in early April 2013.[4] He made his first start for Munster in a United Rugby Championship fixture against Cardiff Blues on 8 February 2014.[5][6] He signed a one-year development contract extension in March 2014.[7] In June 2015, it was announced that O'Mahony would be leaving Munster.[8]
