Longford took the lead on 20 minutes through Thomas Crawley but Maguire equalised six minutes after the break.
The visitors were reduced to 10 men five minutes later when referee Keith Callanan felt Willo McDonagh’s tackle on Maguire was worthy of a straight red card — a decision which infuriated the Town bench as substitute Austin Skelly also received a red card from the Cork referee.
On 66 minutes Peter Keegan gave the Blues the lead when he beat keeper Craig Hyland from close range. But Tony Griffiths restored parity with a deflected free-kick soon after.
Maguire, back from Irish underage duty, then showed his class less than 60 seconds later to put the home side back in front when the teenager skipped past three challenges and drilled a low right-footed shot past helpless keeper Craig Hyland.
The striker was celebrating his hat-trick in the final minute from the penalty spot after he was upended in the area by defender Michael Lee.
It was fitting that he sent the Longford netminder the wrong way to copperfasten Waterford’s best performance at home this season.
WATERFORD UNITED: Holden; Mernagh, Burns, Long, Frost; Higgins, Dempsey, Mulcahy (Breen 90), Phelan; Maguire (Kiely 90), Keegan
LONGFORD TOWN: Hyland; Salmon, Collins (Raynor 84), McDonagh, Lee; Griffiths, Gillespie (Kirby 77), Bayly (Deans 62), James, Crawley; Bermingham
Referee: Kevin Callanan (Cork).
Athlone 0 Mervue United 3
By Kevin Egan
Mervue United comfortably brushed aside Athlone Town in front of a small crowd at Lissywoollen.
The Galway visitors made the breakthrough in the 69th minute when Pat Hoban finished off the simplest of headers from four yards.
Five minutes later Athlone midfielder Shane Dunne was dismissed following his second yellow card and from then on the home side were overwhelmed.
Jean Biansumba tapped home the second goal in the 79th minute and Pat Hoban got his second in injury time when he had ample time to deliver a crisp half-volley past Pat Jennings after good work from Martin Conneely.
ATHLONE TOWN: P Jennings; D Rushe (B O’Mahony), B McCarthy, J Woods, K Williamson; S Relihan (I Sweeney); N Scullion, S Dunne, V Feeney; M McGoldrick (J Conroy), M Sherlock.
MERVUE UNITED: G Hanley; T King, M Conneely, M McSweeney, S Walsh; J Molloy (K Crehan), C McBrien (B Connolly), S Keogh (A Lee), E Nkololo; P Hoban; J Biansumba.
Referee: Rob Harvey.