The game began slowly with both teams trying to feel their way into the game on a pitch in fine condition despite the recent wet weather.
Trinity had the game’s opening chance, as Ronan Hanaphy’s quick throw found Eoghan Vaughan, his flat cross was only half-cleared but quarter-final hero Hanaphy could only fire the follow-up wide.
Limerick were the stronger physically but the Dublin outfit’s skill was making up the difference, and only a crucial intervention by Conor Maguire denied Ayman Ben Mohamed the chance to finish Chris Straessle’s fizzing cross at the back post.
Brian Foley headed a Straessle free-kick over in the 32nd minute, but had more success as his bullet header at the front post from a Vaughan corner fired the Dubliners into the lead.
Apart from a half-chance for Aidan McGrath, UL struggled in the first half and Trinity were good value for their lead at the break.
Both sides had chances early in the second half, with Andrew Cowper putting his body on the line to deny Ben Mohamed before McGrath saw his volley go just wide for the Shannonsiders.
Ben Mohamed should have doubled his side’s advantage but fired wide.
Trinity had calls for a penalty turned down by referee Marc Barry, who saw Conor Maguire and substitute Oisín Pennycook’s collision as a fair shoulder. Limerick suffered a blow as striker Garbhan Coughlan exited with an ankle injury with 20 minutes left.
However, there was to be a twist in the tail, as Trinity striker Ben Mohamed received his marching orders for striking Cormac Cullinane off the ball.
That spurred UL boss Adrian Fitzpatrick to gamble, and he introduced Conor Moran with Oisín Hassett pushing his side forward. The ploy paid off, as Jonathan Hanafin found AJ O’Connor at the back post to power a header past helpless Trinity netminder Dave Minihane with only three minutes left in regular time.
It seemed after that the game would be heading to penalties after a quiet extra installment, but in the dying moments a long Barry Houghton throw was flicked on and McMenamin snuck in at the back post to head home, sparking wild celebrations.
University College Dublin will once again contest the competition’s final, as Gary O’Neill’s 81st minute penalty saw them through 1-0 against an extremely competitive NUI Maynooth side.
Earlier in the day, University College Cork consoled themselves for Wednesday’s exit with a 4-1 demolition of Queens University in the Farquhar Cup; a Dave Walsh double added to by Sean O’Callaghan and Cian Murphy, Ryan Murray with a late Queens consolation. Joining them in tomorrow’s final will be DCU, who beat Mari I 2-1 after extra-time, Alan O’Sullivan with the winner.