He started and finished his career as a player in Ireland but made most of his professional appearances in the English Football League at Stockport County, where supporters elected him as a member of the Hall of Fame.

[20] Despite being appointed as captain upon his arrival, he initially struggled to get into the first-team during the 2001–02 League Championship winning season, and considered a move back to England.

Under Fenlon, Gannon helped the "Shels" to a second-place finish in 2002–03, and also played in the 2002–03 UEFA Champions League qualifying rounds, scoring past Hibernians in a 2–2 draw at the Ta' Qali National Stadium.

[24] This left Dundalk without a manager for their final match of the 2005 season, in which they had already secured a comfortable mid-table position in Ireland's second tier of association football.

Gannon also branded approaches from Manchester City and Liverpool questionable and unprofessional after the two Premier League clubs swooped for youngsters being trained in Stockport County's development squads.

[46][47] Gannon led County to an impressive start to the season following their promotion,[48] and they spent Christmas in the play-off places with the best away record in the division.

[49] The new year started uncomfortably for Gannon; with media speculation linking him with a move away from Edgeley Park, and the club announcing they were in financial difficulty, he took interviews reassuring fans of his intent to stick by County.

[52] On 24 February 2009 Stockport agreed to allow Gannon to hold talks with Brighton & Hove Albion, who were seeking to fill the managerial position left vacant by Micky Adams.

[101] Gannon was confirmed as the manager of Port Vale on 6 January 2011, succeeding Micky Adams at the helm of the promotion chasing club, signing a contract that lasted until summer 2012.

[110] Losing four of his first five games in charge, Gannon was forced to respond to criticism over his team selections and substitutions,[111][112] and announced plans to expand the club's non-playing staff.

[113] On 1 February the players met with the chairman and rumours spread that Gannon had 'lost the dressing room',[114] as comparisons were made with the tenure of Brian Clough in The Damned United.

[116] However, tension remained, as proven at the end of the month when it was reported that Gannon left the team bus on a pre-match journey after rowing with assistant manager Geoff Horsfield.

[118] It was later reported in the national media that Horsfield had requested a day off to deal with a family matter, Gannon then permitted him but subsequently wrote a letter to the board complaining of his conduct.

This came on the same day that local paper The Sentinel ran an editorial that questioned his loan signings, his decision to drop Gary Roberts and branded his tactics and team selections as baffling.

[122] The club informed Horsfield that he would face a disciplinary hearing, whilst Gannon would not be asked to defend his actions and would merely provide a witness statement.

[126] Ugly post-match scenes at the Crown Ground saw Geohaghon confront fans who singled him out for abuse, whilst Gannon was ushered quietly out of the back door.

[130] Hugely unpopular with Vale supporters,[131] his sacking was "quick and with little emotion",[132] whilst his poor man-management skills helped to earn him the dubious honour of having the shortest reign in the club's history.

[134] Gannon himself made a statement on his website declaring his disappointment in the club's decision to terminate his contract after such a short period of time, and also bemoaned that he "wasn't able to add many new players [to the squad]".

[135] The "Valiants" continued to struggle under caretaker manager Mark Grew, winning just two of their remaining ten games, and finishing 2010–11 in eleventh place.

Following the resignation of Dietmar Hamann in November 2011, speculation came of a return to Stockport County for Gannon; when questioned on the possibility, club chairman Peter Snape stated that should he apply for the position then "he'd merit very serious consideration".

[136] In the thirty months since Gannon's departure, the club had seen five full-time managers come and go,[137] suffered relegation into the Conference National and had not won back-to-back games since February 2009.

[139] After losing his first five games in charge, including a 7–0 thumping from Grimsby Town at Blundell Park, Gannon admitted that the club's finances limited his ability to change personnel.

[148][149] Worried by the situation at crisis clubs Darlington and Kettering Town, Gannon cancelled the contracts of veterans Matt Glennon, John Miles, Mark Lynch, and Ryan McCann.

[150] He brought in 23-year-old Telford defender Sean Newton and 18-year-old Sunderland keeper Lewis King on loan,[151][152] whilst signing 19-year-old Lancaster City midfielder Danny Hattersley on a permanent transfer.

[153] The "Hatters" secured their Conference status with two games to spare, after a 1–1 home draw with Braintree Town on 14 April, having lost just twice in the league at Edgeley Park since Gannon's return.

"[160] Fourteen months into his second spell at Edgeley Park and with County sitting in the relegation zone, Gannon was relieved of his duties as Stockport manager on 16 January 2013 following the 3–1 home defeat by Mansfield Town.

[163] He took the club on a successful FA Cup run in the 2015–16 campaign, leaving them as the lowest rank team in the competition when they were knocked out by League Two side Northampton Town in the Second Round.

[166] Gannon started well, taking the team to the Cheshire Senior Cup final and up to ninth in the table with a sequence of seven games unbeaten, including five victories, in the run-up to Easter.

[172] Gannon was named as Manager of the Month award in the National League North for December 2018 after his team rose from the bottom half of the table to the top three and also advanced in the FA Trophy.

[177] On 21 January 2021, with the club sitting fourth in the National League, Stockport parted company with Gannon, the board citing that "the decision is not results based but is centred around culture".