After shining at various age grades with Presentation Brothers College, Cork, including three years with the Senior Cup Team, Deasy joined the Munster Academy in 2007.[1]
He made his Munster debut as a replacement for Ronan O'Gara against Connacht in a Celtic League fixture on 26 December 2009.[2] He caught attention when he replaced an injured Paul Warwick early in a league game against Edinburgh on 19 February 2010. Despite having a mixed night with his place-kicking, he scored a superb individual try when he raced through a gap, chipped ahead and won the race to the try-line.[3] He made his first start for Munster on 19 March 2010 in a league fixture against Scarlets, playing fullback and winning the Man-of-the-Match award.[4] Injuries brought Deasy into Munster's 2009–10 Heineken Cup squad, and he made his European debut in the semi-final against Biarritz on 2 May 2010.[5] He played for Munster A during their 2009–10 British and Irish Cup campaign. His performances for club and province earned a full-time contract with Munster, and Deasy joined the senior Munster squad for the 2010–11 season.
Deasy started at fly-half for Munster A in their 31–12 2011–12 British and Irish Cup Final win against Cross Keys.[6] Less than a week later, Deasy started at Fly-half for Munster in their final league game of the 2011–12 Pro 12 season against Ulster, after Ian Keatley withdrew with a knock. Deasy kicked 9 points in the 36–8 win, helping to secure a place in the play-offs for Munster.[7][8] During the 2011–12 season, Deasy established himself as Munster's third-choice fly-half.
Deasy cites his coaches at PBC as the biggest influence on his career.[9]
It was announced on 14 May 2013 that Deasy would be leaving Munster, to an as yet unannounced club.[10]
Upon leaving Munster, Deasy did not seek another professional contract, and instead signed for amateur club Lansdowne Football Club, with whom he won the All-Ireland League in 2015. Deasy is now Lansdowne's all-time top scorer in the All Ireland League having amassed more than 600 points in the league for Lansdowne