Byrne signed for Nottingham Forest in July 2007 from Dublin schoolboy side, Crumlin United after unsuccessful trials at Leicester City and Stoke City. He was named as a substitute in Forest's 1–1 draw at home to Walsall in March 2008. He then made his professional debut for Forest as a 70th-minute substitute on 5 April 2008 in a 3–1 victory against Cheltenham Town. With the Reds gaining promotion, manager Colin Calderwood commented that he would be given the chance to impress next season after Forest's promotion to the Championship in the upcoming 2008–09 season.
On 13 March 2009, Byrne was sent on loan to Conference National side Burton Albion for the remainder of the season, featuring in seven of the remaining eleven fixtures before being promoted, as champions, to the Football League.
On 20 July 2009, Byrne joined Conference National side Rushden & Diamonds on an initial six-month loan, but eventually signed on for the rest of the season. The Irishman made 44 appearances with Rushden & Diamonds in 2009–10 scoring seven goals in the process, including the equaliser in Diamonds' Conference Premier Semi-Final First Leg vs Oxford United. He received two awards at the end-of-season presentation night. He left Nene Park at the end of his loan spell at the season's end.
At the start of the 2010–11 season, he joined Barnet until 1 January 2011 when his loan spell was cut short due to injury. Byrne returned to Barnet on loan in March 2011. After his release from Nottingham Forest in June, Byrne signed a two-year deal with Barnet. Byrne then consolidated his position within the Barnet first team, playing a central role in midfield, though at times, he was shifted on to the right wing. He scored in the first game of the season on 6 August, against Morecambe at the Globe Arena, scoring in the 76th minute in a 1–0 win for Barnet. He next scored on 19 November, with a 98th-minute goal against Bristol Rovers, sealing a 2–0 win at the Memorial Stadium. He scored again in the League on 21 January, with a 48th-minute equaliser against Northampton Town at Sixfields Stadium. Next, he scored against Rotherham United on 3 March, with the opening goal in the 7th minute at Underhill. Barnet went on to draw the game 1–1. Finally, he scored a pivotal goal on the final day of the season away against Burton Albion, a game which Barnet needed to win to seal their spot in the Football League – he scored the opening goal in the 6th minute. Barnet would go on to win the game 2–1, thus maintaining their Football League status. He became a popular figure among Barnet fans for his hardworking attitude and tough tackling. In the following season, Byrne's position was shifted more prominently on to the right wing, fitting in with new player-manager Edgar Davids' 4–5–1 formation with two wingers, and three central midfielders. Byrne was one of few players who stayed at Barnet following the previous season. He would go on to play a prominent role for Barnet, scoring 2 penalties in the League, with a total of 3 goals in the League – the joint 4th highest goal tally for Barnet. Due to a shortage of right backs following injuries to Barry Fuller, and a previous long term injury to Danny Senda, Byrne also filled in at right back for Barnet, with moderate success. He made 40 appearances during the League. At the end of the 2013–14 season, Byrne was released by Barnet. In total, he played 166 times for the Bees, scoring 19 goals.
In July 2014Justin Edinburgh signed Byrne for Newport County. Edinburgh had been Byrne's manager at Rushden and Diamonds. He made his debut for Newport against Wycombe Wanderers on 9 August 2014. Byrne scored his first goal for Newport in the 1–0 win over Southend United on 21 October 2014. He was offered a new contract by Newport at the end of the 2015–16 season but the offer was withdrawn and he was released after he failed to accept the contract by the deadline of 10 June 2016.
On 15 June 2016 Edinburgh again signed Byrne, this time for Gillingham on a two-year contract. He scored his first goal for the club in the 2–1 win over Premier League side Watford in the second round of the 2016–17 EFL Cup. He signed a new two-year contract with the club in June 2018, having been named the Player of the Season at the conclusion of the 2017–18 campaign. Byrne was released by the club at the end of the injury-stricken 2019–20 season. In total, he made 157 appearances in all competitions for the Kent side, scoring 11 goals.
Newport County Player of the Season, Player's Player of the Year, Goal of the Season, Supporters Club Away Player of the Year, Supporters Club Player of the Year
Gillingham Player of the Season, Goal of the Season, Away Player of the Year, Players' Player of the Year
