Casey made his debut for Munster A on 11 December 2010, coming on as a replacement against Melrose in the 2010–11 British and Irish Cup. He was included in Munster squad for the 2012–13 Heineken Cup when it was announced on 28 September 2012. Casey secured a development contract with the senior Munster squad for the 2013–14 season in April 2013. He made his senior Munster debut on 8 December 2013, coming on as a replacement against Perpignan during Round 3 of the 2013–14 Heineken Cup. In January 2014, Casey signed a new two-year contract with Munster, which will see him remain with the club until at least June 2016. Casey started in Munster's first European Rugby Champions Cup game against Sale Sharks on 18 October 2014. He started against Saracens in Round 2 of the Champions Cup on 24 October 2014. Casey scored a try in the 38–12 away win against Dragons on 21 November 2014. He started against Clermont Auvergne on 6 December 2014. Casey also started in the reverse fixture against Clermont on 14 December 2014, scoring a try. He started against Saracens on 17 January 2015. Casey started the final pool game against Sale Sharks on 25 January 2015. He scored a try in Munster's 42–20 win against Connacht on 28 March 2015. Casey won the 2015 Munster Rugby Young Player of the Year award in April 2015. In February 2016, it was announced that Casey had signed a two-year contract extension with Munster. Casey left the province in January 2018, having secured a contract with a French club. He won the Vodafone-sponsored Medal for Excellence at the 2018 Rugby Players Ireland Awards in May of that year. The award, for which players are nominated by their peers, is given to an "individual who has shown exceptional commitment to the game of rugby both on and off the field".
Grenoble
Casey joined French Pro D2 side Grenoble as a medical joker for Arnaud Héguy in late January 2018. Casey's contract was extended in March 2018, meaning he will stay with Grenoble for the 2018–19 season. Casey announced in April 2019 that he would retire from rugby at the end of the 2018–19 season.
Ireland
Casey has represented Ireland Schools and Ireland Under-20.
