The Pilgrims were too strong for Sheffield United's under-21s at the Jakemans Community Stadium and led 1-0 at half-time through a rare Dylan Hill strike.
Trialist D added a penalty and Alex Lankshear a thumping header - before Donnelly's 14th minute treble added gloss to the scoreline for Graham Coughlan's side.
United: Rhys Lovett (Zach Marsh 75), Michael Kelly (Ben Grist 61), Connor Teale, Zak Mills, Greg Sloggett, Rocco Coyle (Billy Bradley 69), Dylan Hill, Frankie Maguire (Luca Alonzi 75), Alex Lankshear (Arjan Raikhy 69), Jordan Richards (Manni Norkett 61), Trialist D (Kieren Donnelly 61). Sub (not used): Bobby Jones (gk).
Referee: Emily Heaslip.
Attendance: 609 (51 visitors).
PIC: Chris Bray.