A sharper Longford Town had this relegation tussle sewn up at half-time, adding to Monaghan's survival problems in the process.
Monaghan, who had shown good form in recent games, disappointed deeply. The opening minutes gave promise of a lively encounter but Monaghan seemed to lose confidence following the departure of centre back Robbie Brunton, injured in the third minute.
Thereafter they were in continuous trouble and Longford's half-time 2-0 advantage would have been greater but for a couple of fine saves by United goalkeeper, Gareth Downey. In the eighth minute he went full length to turn away Stewart Holt's daisy-cutter.
Monaghan could have been in even more trouble in the 17th minute when Fergal Coleman missed a great chance from close range from Sean Prunty's cross. Alan Kirby twice went close for Longford before Paul McNally headed them in front in the 29th minute from a corner by Prunty.
In the 36th minute, Downey made a great save from a Holt free-kick but Monaghan were playing second fiddle at this stage, Darren McKenna having the best chance of the half but failing to control the ball in the Longford goalmouth. The goal that clinched the issue came in the 42nd minute when Coleman scored from close range.
Monaghan were further disrupted in the second half when their other centre back, Brendan Place, retired injured. The half was much more closely fought, however, and Monaghan's makeshift defence held out well against a Longford attack in which Eric Lavine posed them most problems. On the other hand, although Monaghan did their share of the attacking they rarely troubled goalkeeper Stephen O'Brien.
MONAGHAN UNITED: Downey; White, Place, Brunton, McDonnell, Mieir, Shiels, McGrath, King, McKenna, Vaughan, Subs: Scully for Brunton (3 mins). Murphy for Place (half-time).
LONGFORD TOWN: O'Brien; Murphy, S Byrne, McNally, Holt, Kirby, Reynolds, Prunty, W Byrne, Lavine, Coleman, Subs: Perth for Reynolds (67 mins), Zellor for Coleman (86 mins), Gavin for Kirby (88 mins).
Referee: J Breen (Dublin)
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