Dempsey first came to prominence as a hurler with Ardscoil Rís in Limerick. Having played in every grade as a hurler, he was an unused substitute on 27 February 2011 when Ardscoil Rís retained the Harty Cup title after a 3-19 to 0-03 defeat of C.B.S. Charleville in the final.[2]
Dempsey joined the Na Piarsaigh club at a young age and played in all grades at juvenile and underage levels, enjoying championship success in the under-16, minor and under-21 grades. He made his senior championship debut for the club as a seventeen-year-old in 2013.
On 6 October 2013, Dempsey was at right wing-forward when Na Piarsaigh defeated Adare by 0-14 to 0-12 to win the Limerick Senior Championship.[3] He retained his place in the half-forward line for the subsequent Munster Championship, which culminated with him winning a provincial medal after a 4-14 to 0-08 defeat of Sixmilebridge in the final.[4]
Dempsey won a second Limerick Championship medal on 11 October 2015 after a 1-22 to 4-12 defeat of Patrickswell in the final.[5] Later that season he won a second Munster Championship medal after a 2-18 to 2-11 defeat of Ballygunner.[6] On 17 March 2016, Dempsey won an All-Ireland medal when Na Piarsaigh defeated Ruairí Óg by 2-25 to 2-14 in the final.[7]
On 15 October 2017, Dempsey won a third Limerick Championship medal when Na Piarsaigh defeated Kilmallock by 1-22 to 2-14 in the final.[8] He later won a third Munster Championship medal when Na Piarsaigh defeated Ballygunner by 3-15 to 2-10 in the final.[9] On 17 March 2018, Casey was named at left wing-forward but played at midfield when Na Piarsaigh were defeated by Cuala in the All-Ireland final.[10]
On 27 October 2018, Dempsey won a fourth Limerick Championship medal following Na Piarsaigh's 2-22 to 3-10 defeat of Doon.[11]
Inter-county
Minor and under-21
Dempsey first played for Limerick at minor level. On 23 July 2013, he scored two points when Limerick won their first Munster Championship title in 29 years after a 1-20 to 4-08 defeat of Waterford in the final.[12]
Dempsey subsequently joined the Limerick under-21 hurling team in 2015 and won a Munster Championship medal after a 0-22 to 0-19 defeat of Clare in the final.[13] On 12 September 2015, Dempsey was at left wing-forward when Limerick defeated Wexford in the All-Ireland final.[14]
Intermediate
In 2016, Dempsey joined the Limerick intermediate hurling team. On 6 July 2016, he was at right corner-forward when Limerick were defeated by Clare in the Munster final.[15]
Senior
Dempsey joined the Limerick senior hurling team in January 2016.[16] He was an unused substitute throughout the entire season.
On 19 August 2018, Dempsey was a member of the extended panel when Limerick won their first All-Ireland title in 45 years after a 3-16 to 2-18 defeat of Galway in the final.[19]
On 31 March 2019, Dempsey was named on the bench for Limerick's National League final meeting with Waterford at Croke Park. He collected a winners' medal as a non-playing substitute in the 1-24 to 0-19 victory.[20]