The second half saw Harps step up a gear and finished much the stronger outfit against a Monaghan side who completely fell to pieces after the break.
Number two was added after referee Derek Tomney awarded the hosts a penalty when Darren Meenan was upended by Monagahan goalkeeper Tadhg Murphy.
Centre-half Shane Bradley assumed responsibility for the spot-kick as he confidently stepped up to convert from twelve yards.
On 71 minutes, Kerrykeel native Davitt Walsh fired home his eighth goal of the campaign to seal the tie as a contest and Matthew Crossan put the tin hat on a man-of-the-match performance with goal number four after substitute Barry Mulligan did the spadework.
Mulligan was making his competitive debut for Harps last night while Darren Meenan made his first start for the club on what was a fantastic night for those in what was again a relatively poor home attendance.
Next up for Harps is a home tie against Athlone Town in the FAI Cup next Saturday night.
Finn Harps: James Gallagher; Shaun McGowan, David Bell, Eloka Asokhu, Shane Bradley; Michael Funston, Barry Curran, Jonathan Minnock, Matthew Crossan; Darren Meenan, Davitt Walsh. Subs: Ryan Mc Gavigan (for Curran); Barry Mulligan (for Walsh); Shane O'Connor (for Funston).
Monaghan United: Tadhg Murphy; Brian Gannon, Michael Kerr, Thomas Mc Gauley, Paul Whelan; David Lee, Austin Mc Cann, Jimmy Fullam, Paul Smith; Robbie Smith, Robbie Farrell. Subs: Finbar Heffernan (for P.Smith); Stephen Finnegan (for Gannon).
Referee: Derek Tomney
Finn Harps Dot Com Man-of-the-Match: Once more there were several candidates for this with Darren Meenan and Davitt Walsh both turning in impressive performances in the Harps attack, while Shane Bradley was again a colossus at the back and chipped in with the second goal from the spot.
This week's award, however, goes to Matthew Crossan. The young Killea man was started in the unfamiliar role in the centre of the four-man midfield and he was excellent alongside Michael Funston.
Crossan's work rate, passing and tackling were flawless and he capped off what was a fine evening's work with the fourth goal, which was his second strike in Harps colours this term.