O'Mahony made his debut for Munster A on 27 November 2009, replacing Simon Zebo in a British and Irish Cup game against Bristol and scoring a try. He was part of the Munster A team that won the 2011–12 British and Irish Cup, scoring a try and conversion against Neath RFC. He was promoted to a development contract with the senior Munster squad for the 2013–14 season. O'Mahony made his debut for the senior Munster team on 19 April 2013, starting against Newport Gwent Dragons in a Pro 12 fixture and scoring a try. He scored the winning try in Munster's 16-10 win against Scarlets on 21 December 2013. O'Mahony signed a one-year development contract extension in March 2014. He signed a two-year contract extension in February 2015.