[3] A midfielder, he returned home and signed for Brian Kerr's St Patrick's Athletic in 1987[3] and made his League of Ireland debut on 13 September 1987, against Shelbourne at Harold's Cross Stadium.

Fenlon signed for Linfield in January 1994, a club supported mainly by fans drawn from Belfast's Protestant community, becoming the first Dublin catholic to do so.

[3] Whilst playing for Shels he suffered a triple fracture of the shin in a match against Rovers in Tolka Park on 12 February 1999.

He recovered from this to help fire Shelbourne to the league and cup double the following season, and secure a second PFAI Player of the Year award.

[9] Following this amazing run, and participation in the First Round of the UEFA Cup against Lille OSC, Fenlon was rewarded with a contract extension.

This was followed by reports linking Fenlon to the vacant hot-seat at Derry City left by Stephen Kenny, who had departed the Brandywell Stadium to take up the managers position at Scottish side Dunfermline Athletic.

Fenlon resigned from his position as Shelbourne manager on 5 December 2006[11] and the reports proved true when Derry concluded an agreement with him.

Scottish Premier League club Dundee United attempted to appoint him as their manager in January 2010, although their compensation offer of £90,000 was rejected, with Bohemians looking for over £200,000.

[17][18][19] Unusually, what would be Fenlon's debut game as manager away to Motherwell was abandoned at half time,[20] when one of the Fir Park floodlights caught fire.

[22] With the club still struggling, Fenlon decided to shake up the squad in January 2012 by releasing some players and replacing them with loan signings, including James McPake.

Furthermore, a late goal by Leigh Griffiths gave Fenlon's side a 2–1 victory against Aberdeen and a place in the 2012 Scottish Cup Final.

Wins against Aberdeen, Kilmarnock and Falkirk meant that Hibs reached a second successive Scottish Cup final, which they lost 3–0 to Celtic.

[30] The 2013–14 season began with a humiliating 9–0 aggregate defeat to Swedish side Malmö in the second qualifying round of the Europa League, including a 7–0 reverse at Easter Road in the home leg.

[30] A run of just one defeat in nine matches left Hibernian safely in mid-table and eased that pressure, but some Hibs fans protested against Fenlon after the team lost a League Cup quarter-final against Hearts.

[35] Fenlon left his post at Waterford in February 2018 to work with the chairman of the club Lee Power as a consultant on his worldwide projects.