Beirne spent most of his career at Leinster playing for their A team and he earned 10 caps for them, making 9 appearances in the British and Irish Cup.
[11] Beirne made his Champions Cup debut for Munster on 13 October 2018, earning the Man-of-the-Match in the provinces opening 2018–19 pool 2 fixture against English side Exeter Chiefs, which ended in a 10–10 draw in Sandy Park.
[8][16] Beirne suffered a sustained a fractured ankle in Munster's 15–6 defeat away to English side Saracens in round 4 of the 2019–20 Champions Cup on 14 December 2019, and subsequently had to undergo surgery for the injury in January 2020.
[18] When regular captain Peter O'Mahony was injured in the warm-up ahead of Munster's 2021–22 United Rugby Championship round 10 fixture at home to provincial rivals Ulster on 8 January 2022, Beirne, who had originally been named on the bench, was promoted to the starting XV in O'Mahony's place and also to the captaincy, leading Munster for the first time and earning the player of the match award in the 18–13 win.
[19] Beirne signed a new three-year central contract with Munster and the IRFU in February 2022, a deal that will see him remain with the province until at least July 2025.
[20] Beirne earned his 50th cap for Munster in their 24–17 win against provincial rivals Connacht in round 8 of the 2022–23 United Rugby Championship on 26 November 2022.
[21] An ankle injury sustained whilst on international duty during the 2023 Six Nations required surgery, ruling Beirne out for 12 weeks.
[32] Beirne was selected in the 31-man Ireland squad for the 2019 Rugby World Cup,[33] having featured in the warm-up matches against Italy,[34] England[35] and Wales.
[53] After Iain Henderson was injured minutes before kick-off, Beirne was promoted from the bench to start against Argentina in Ireland's final fixture of the series on 21 November, scoring a try in their 53–7 home win.
[66] Beirne was named in World Rugby's Men's 15s Dream Team of the Year at their annual awards held in Monaco in November 2022.
[67] Beirne continued to start for Ireland in their opening fixture of the 2023 Six Nations Championship against Wales on 4 February, being replaced by Ulster's Iain Henderson during the second-half of Ireland's 34–10 away win,[68] before also starting in the 32–19 win against France in round two on 11 February, though Beirne left the field with an ankle injury during the second-half.