[1] He previously played for Shamrock Rovers (over 2 spells), St Patrick's Athletic and Dundalk and has won 11 League of Ireland Premier Division winners medals.

Gannon has featured regularly in European competition for Dundalk and was part of their team that became only the second Irish side ever to reach the group stage of the UEFA Europa League in August 2016.

[11][12][13] 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.

[14] 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[15] and also a place in the PFAI Premier Division Team of the Year.

[17] The following season, Gannon appeared in all but one of Dundalk's Premier Division matches as they retained their league title,[18] scoring once in a 2–0 win over Shamrock Rovers.

[20] 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.

[citation needed] Gannon signed back for The Hoops in December 2020 [24] 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 Professional appearances – correct as of 30 August 2024.