[1] Kilcoyne joined the Munster Academy at the start of the 2010–11 season, after impressing in the 2009–10 British and Irish Cup campaign.

[4] He also started in the 2011–12 British and Irish Cup Final on 27 April 2012, which Munster A won after beating Cross Keys 31–12 in Musgrave Park, Cork.

[17] Kilcoyne started in Munster's Heineken Cup quarter-final against Toulouse on 5 April 2014, scoring a try in the 47–23 win.

[21] He started against Sale Sharks in Munster's first European Rugby Champions Cup game on 18 October 2014, scoring a try.

[24] Kilcoyne started Munster's opening pool game of the 2015–16 European Rugby Champions Cup against Benetton on 14 November 2015.

[30] On 9 September 2017, Kilcoyne made his 100th appearance in the United Rugby Championship when he came off the bench against South African side Cheetahs in round 2 of the 2017–18 Pro14.

[31] Kilcoyne was Man-of-the-Match in Munster's 14–7 win against Racing 92 in round 2 of the 2017–18 European Rugby Champions Cup on 21 October 2017.

[32] Kilcoyne earned his 150th cap for Munster on 7 April 2018, coming off the bench and scoring a try in the provinces' 39–22 away win against Southern Kings.

[34] Kilcoyne earned his 200th cap for Munster in their 34–20 home win against Edinburgh in round 12 of the 2021–22 United Rugby Championship on 18 February 2022.

[35] A neck injury sustained during Ireland's final 2022 Six Nations fixture in March eventually required surgery in April, ruling Kilcoyne out for up to 12 weeks.

[38] Kilcoyne made his Ireland debut on 10 November 2012, as a temporary replacement for Cian Healy in the test against South Africa.

[44] Kilcoyne made his Six Nations debut on 2 February 2013, coming on as a replacement in Ireland opening 30–22 win against Wales.

On 7 November 2016, Kilcoyne was added to the senior Ireland squad for the 2016 end-of-year rugby union internationals.

[68] He was also selected in the squad for the 2017 Summer Tour against the United States and Japan,[69] coming off the bench in all three tests.

[73][74] Kilcoyne was selected in the 31-man Ireland squad for the 2019 Rugby World Cup,[75] having featured in the warm-up match against Wales.

[81] Retained by new head coach Andy Farrell in his squad for the 2020 Six Nations Championship, Kilcoyne was used as a replacement in Ireland's 19–12 opening win against Scotland on 1 February 2020,[82] their 24–14 win against defending champions Wales on 8 February,[83] and their 24–12 defeat against England,[84] before the tournament was suspended due to the COVID-19 pandemic.

[100] Kilcoyne started for Ireland in their opening 2023 Rugby World Cup warm-up match against Italy on 5 August, scoring a try in their 33–17 win.