[4] After beginning his career with St. Joseph's,[5] Byrne joined Manchester United in July 2011, entering the club's academy as a first-year scholar.
[14] At the start of the 2018 season, he joined League of Ireland Premier Division side Dundalk on loan before signing permanently for the club on 9 July 2018.
[16] On 30 January 2019, it was announced that Byrne had signed for Bohemians for the 2019 season; he had scored a penalty in a 4–0 friendly win against Longford Town the previous night.
[23] He missed the team's trip to Turkey in March 2011[24] after being selected for the under-17s' 2011 UEFA Under-17 elite round matches against Denmark, Greece and Latvia, but returned to captain the side in a double-header at home to Belgium in April 2011,[25][26] his final under-16 appearances.
[27] His impressive performances in Hungary meant he was retained in the squad for the 2011 UEFA Under-17 qualifiers against Norway, Albania and group hosts Malta.
[38][39] After a training camp in Portugal in February 2012, Byrne was again selected for the under-17 squad for the elite round matches against Serbia, the Netherlands and Albania in March 2012.
[40] He started all three games, but Ireland lost all three by a one-goal margin and finished bottom of their group,[41][42][43] bringing Byrne's under-17 career to a close.
[52] Byrne continued in the under-19s for the 2013–14 season, captaining the side for a friendly against Norway on 13 August 2013,[53] before scoring after coming off the bench against the same opposition two days later.
[54] After another friendly double-header against Slovenia in September 2013,[55] Byrne was named in the Ireland squad for their 2014 UEFA Under-19 qualifiers against Azerbaijan, Sweden and Bosnia and Herzegovina in October 2013,[56] only for an injury suffered while playing for the Manchester United youth team to rule him out of the entire campaign.