The Munster Senior League side started the stronger with some swift attacking football and they deservedly jumped in front after just seven minutes.
The Long brothers combined in attack for Danny to pass into the path of Roy, who thumped a powerful shot past home goalkeeper David Browne from 12 yards.
Despite having more of the ball United failed to add to their lead and it was Ramblers, with a Shane O’Connor free kick in the 72nd minute, who came close to levelling it up.
The 20 yard effort from the former Ipswich Town and Limerick player beat the defensive wall and goalkeeper Daniel O’Leary, but flew wide of the target.
But the home side struck an 85th minute equaliser when a Ryan Goldsmith cross was powerfully headed home at the far post by Rob Waters to send the game into extra-time.
Avondale were awarded a penalty kick when substitute Simon Holland was brought down in the area and another replacement Ray Lally confidently tucked home the 93rd minute spot kick against one of his former clubs.
Avondale, who have lost two previous finals, to Cork City (2005) and Waterford (2013), now meet Limerick in the decider as they attempt to win the provincial trophy for the first time.
COBH RAMBLERS: Browne; R McCarthy, E Murphy, Frahill, O’Connor; Goldsmith, D A Murphy, Waters, Kenny; Rocks, Stanton.
Subs: S McCarthy for Rocks (inj 10),Curran for Stanton (h/t), Bhandarkar for S McCarthy.
AVONDALE UNITED: O’Leary; Kiely, Horgan, Cahill; Lougheed, Comerford, Mulconry, Caulfield, Connolly; D Long, R Long.
Subs: Lally for Cahill (76), Holland for Connolly (80), Warren for D Long (107).
Referee: Graham Kelly (Cork).