[4] As a player Keely started with Home Farm and then signed for St. Patrick's Athletic under Barry Bridges who appointed him club captain.
In the return leg in front of a packed Oriel Park with Keely as captain, Dundalk missed a last-minute opportunity to win the tie on away goals and the game finished 0–0 [1].
[1] However, after just two months at Belfield Jim McLaughlin (football manager) persuaded Keely to sign for Shamrock Rovers.
When McLaughlin left at the end of the 1985/86 season to manage his home club Derry City Dermot was appointed player-manager on 16 May.
Keely was quoted years later as saying: I didn't realise the depth of feeling about Glenmalure Park and all I wanted to do was win the League for Shamrock Rovers.He resigned after the last game of the season on 8 April 1988.
After managing Athlone Town he took over the reins at Home Farm Everton where he won the League of Ireland First Division Shield in 1998.
In the calendar year 1996 Keely managed four clubs: Dundalk, Finn Harps, Athlone Town and Home Farm.
He also guided Shels to victory over Macedonian side Sloga Jugomagnat in the first round of the 2000–01 UEFA Champions League qualifiers.
In his second season in charge, Shelbourne lead the division going into the final game, and needed a home win against mid-table Limerick to gain promotion, but were denied by a 92nd-minute equaliser, and so were overhauled by Dundalk, who won away to Kildare County.
[10] Keely has also been working as a Maths and English teacher in De La Salle College Churchtown for over 20 years until he retired at Christmas 2011 he also has a weekly column in the Irish Sun.