In 2010 Gilroy was promoted to the Ulster Rugby first team. Gilroy scored 8 tries during the 2010–11 Celtic League. Gilroy burst onto the scene and scored his first two tries of the 2010–11 season in a match against the Cardiff Blues which Ulster won 37–15. Gilroy again scored twice the next game against Benetton Treviso when Ulster rugby won 19–9. He then made his tally to 5 tries when he scored against Leinster rugby but it was not good enough and Leinster won 30–13. He was ruled out for two games with an injury but scored a try against Cardiff Blues to make him known a young talent. He scored once again in the 23–10 win over Aironi. During the 2010–11 season, Gilroy's club, Dungannon RFC, came fifth. In 2011–12 he scored his first try against Aironi, and then scored against Connacht Rugby in a 22–3 win. He then scored in a 30–12 win over Newport Gwent Dragons to have his Pro12 try number at 3. He also scored 3 times in the Heineken Cup. He scored tries in pool play against Aironi and Leicester Tigers. Gilroy made the news with the try that gave Ulster a 22–16 win in the quarterfinals against Munster at Thomond Park to send Ulster through to the Heineken Cup semi final. This try won him the IRUPA award in 2012. He also played in Ulster's 42–14 loss to Leinster in the 2012 Heineken Cup final. Gilroy was also nominated for Irish Young Player of the Year for the 2011–12 season, but that award went to Munster back row Peter O’Mahony. He was also named Ulster young player of the year. Dungannon RFC made a big improvement in 2011–12 season finishing in third place. Early during the 2012–13 season, Gilroy signed a three-year contract extension to remain with Ulster until summer 2016. In February 2015 he earned his 100th cap for Ulster in a 43–3 win over Treviso, scoring two tries in a Man of the Match performance.
Ireland
During the Ireland Six nations campaign in 2012, Gilroy was called up to train with the six additional youngsters alongside Fionn Carr, Jack McGrath, Ian Nagle, Peter O'Mahony and Tiernan O'Halloran. On 29 May 2012, Gilroy was selected for the 2012 mid-year friendly with the Barbarians. This was his first game for Ireland. Gilroy scored his first international try on 15 minutes. Gilroy later put Ireland ahead with a try on 65 minutes. The Barbarians went on to win the match. On 17 November 2012, Gilroy scored a hat-trick of tries for Ireland in a non-test match against Fiji at Thomond Park. On 24 November 2012, Gilroy was selected for the test against Argentina, scoring the opening try on his test debut in a 46–24 win. He was selected in the Ireland team to face Wales in the opening match of the 2013 Six Nations Championship in Cardiff. On 11 Feb 2017, Gilroy came on as a substitute for Ireland against Italy in Rome and he scored a hat-trick of tries.
