Byrnes joined the Patrickswell club at a young age and played in all grades at juvenile and underage levels. On 23 October 2016, Byrnes won a Limerick Hurling Championship medal after scoring six points from centre-back in Patrickswell's 1-26 to 1-07 defeat of Ballybrown in the final. Byrnes lined out in a second Limerick Championship final on 6 October 2019. Playing at centre-back, he scored two points, including a long-range free, and collected a second winners' medal following the 1-17 to 0-15 defeat of Na Piarsaigh.
Inter-county
Minor and under-21
Byrnes first played for Limerick at minor level in 2012, in a season which ended with a defeat by Clare in the Munster Championship semi-final. Byrnes subsequently joined the Limerick under-21 hurling team. He made his first appearance on 4 June 2014, however, he was sent off after receiving two yellow cards in a 2-20 to 1-14 defeat by Clare. In his second season with the team Byrnes won a Munster Championship medal after a 0-22 to 0-19 win over Clare in the final. On 12 September 2015, Byrnes was at right wing-back when Limerick defeated Wexford by0-26 to 1-07 in the All-Ireland final. He ended the season by being named on the Bord Gáis EnergyTeam of the Year.
Senior
Byrnes made his first appearance for the Limerick senior hurling team on 13 February2016 in a 2-23 to 0-15 defeat of Wexford in the National Hurling League. Later that season he made his first championship appearance in a 3-12 to 1-16 defeat by Tipperary in the Munster Championship. In April 2017, Byrnes sustained a knee injury which rules him out of the championship. On 19 August2018, Byrnes scored a point from right wing-back when Limerick won their first All-Ireland title in 45 years after a 3-16 to 2-18 defeat of Galway in the final. Later that day he was named on The Sunday Game Team of the Year. Byrnes ended the season by being nominated for an All-Star Award. On 31 March 2019, Byrnes was selected at left wing-back for Limerick's National League final meeting with Waterford at Croke Park. He collected a winners' medal following the 1-24 to 0-19 victory. On 30 June 2019, Byrnes won a Munster Championship medal after scoring three long-range frees from right wing-back in Limerick's 2-26 to 2-14 defeat of Tipperary in the final.
