Corner-forward McMonagle had his first goal in the opening half before the sides retired level (1-4 to 0-7) at the interval.
But the sharper Offaly outfit were nine points clear within 10 minutes of the restart. McMonagle netted his second goal four minutes after the break and completed his hat-trick with seven minutes remaining.
Scorers for St Mary’s, Edenderry: C McMonagle 3-1, G Leech 0-6, 2fs, S Doyle 0-3, C McNamee 0-2, R McNamee, S Moriarty, M Nolan 0-1 each.
Scorers for Dundalk Colleges: C Byrne 0-7, 5fs, B McLaughlin 0-2, R Carr, R Quigley 0-1 each.
ST MARY’S, EDENDERRY: C Hurley; L Flynn, D Flynn, M Kearney; S Moriarty, P Foy, J Dowdall; D Grehan, C McNamee; R Caffrey, M Nolan, G Leech; C McMonagle, S Doyle, R McNamee.
Subs: I McKeown for R McNamee, (53), G Blong for Leech (54), A O’Connell for Dowdall, S Haslam for Nolan, M Clear for Doyle (all 56).
DUNDALK COLLEGES: S Murphy; C McMenamin, D Smith, B Faulkner; L O’Leary, J Stewart, J Arrowsmith; C Giggins, C Noonan; B McLaughlin, C Doyle, C McKeever; R Carr, C Byrne, R Quigley.
Subs: D Maguire for Noonan (34), D Bannon for Arrowsmith (36), R Curran for Faulkner (47), K McDonnell for Quigley (49), C Lenehan for O’Leary (53).
Referee: G McCormack (Dublin).