Shamrock Rovers Gannon made his senior debut for Shamrock Rovers in February 2011, coming on as a half-time substitute for
Craig Sives in a
Leinster Senior Cup game away to Dundalk. His league debut came on 12 September when he started in a 1–1 draw with
St Patrick's Athletic, his only league appearance of the 2011 season. Gannon was part of the Shamrock Rovers squad that became the first ever Irish side to reach the group stage of European competition when they knocked out
Partizan Belgrade in the
Europa League 4th qualifying round, however he made no appearances in qualifying and was cut from the 25-man squad for the group stage. In 2012, Gannon moved closer to the Shamrock Rovers first team, making 8 league appearances and playing in 7 cup games, where he was mostly deployed as a right-back. He was part of the Shamrock Rovers team that won the Leinster Senior Cup and scored three times for the club in cup competition. At the end of the season he was released by new manager
Trevor Croly.
St Patrick's Athletic Gannon signed for
Liam Buckley at St Patrick's Athletic on 27 November 2012. He played 13 league games for St Pat's as they went on to win the
2013 League of Ireland Premier Division, featuring mostly as a central midfielder. Although Gannon was kept from playing in his favoured position by right-back
Ger O'Brien, he later stated that he learned a lot from the experience of playing in the same team as him.
Dundalk On 6 November 2013, Gannon linked back up with his former Shamrock Rovers manager
Stephen Kenny at Dundalk, becoming Kenny's first new signing for the 2014 season. In his first season at the club, he was part of a defence that kept sixteen clean sheets on their way to winning the league title on the final day of the season. Gannon played in all thirty-three league games and scored two goals in wins over
Drogheda United and his former club, St. Patrick's Athletic. His goal against the latter included a 36-pass build-up involving all ten outfield players and was widely regarded as one of the league's goals of the season. Gannon also won the
League Cup appearing in a 3–2 defeat of Shamrock Rovers in the final and made his European debut in July 2014 against
Jeunesse Esch. Gannon's reliability, willingness to attack, and form over the course of the season earned him comparison to former
Manchester United full-back
Denis Irwin and also a place in the
PFAI Premier Division Team of the Year. After the season ended, Gannon turned down the opportunity to go on trial to
Millwall in favour of signing a new contract with Dundalk. The following season, Gannon appeared in all but one of Dundalk's Premier Division matches as they retained their league title, scoring once in a 2–0 win over Shamrock Rovers. He played in both
UEFA Champions League second qualifying round games as Dundalk were eliminated by
BATE Borisov. Gannon also won the
FAI Cup, going off injured with an ankle ligament injury in the 43rd minute of the final. He celebrated the winning goal by limping down the touchline on crutches and waving them in the air towards the fans. Following the end of the season, he was again rewarded with a place in the PFAI Team of the Year for the second year in-a-row. Gannon returned from his injury to appear in all 6 of Dundalk's UEFA Champions League qualifier games in 2016 as they became the first ever Irish side to reach the
play-off round. Gannon received significant exposure due to this achievement, appearing on
The 7 O'Clock Show on
TV3 and
RTÉ's
Saturday Night with Miriam in August 2016. He regards the European qualifying games as the stage where League of Ireland players are truly judged, and considers the
third qualifying round victory over BATE Borisov as the highlight of his career to date. 2017 was a more challenging year for Dundalk with rivals Cork City doing the League and FAI Cup double. Gannon signed a new three-year contract with Dundalk in November 2017, and was a key part of the team that retained the league title in 2018. He was also voted onto the PFAI Team of the Year for the fifth year in a row.
Return to Rovers Gannon signed back for The Hoops in December 2020 During his three seasons at the club he played in 17 European ties most notably in the
2022–23 UEFA Europa Conference League group stage ==Career statistics==