All-Star forward Liam Watson grabbed a sensational equalising goal with the last puck of the game to secure the replay after the Connacht Champions had taken control of the contest with an extra-time goal of their own.
The sides went toe-to-toe for the full 100 minutes in a breathless encounter, with each team holding the lead on multiple occasions over the course of the game and neither leading by more than a goal at any stage.
It was St Thomas' who started brightest, rattling off three unanswered scores to take an early lead, but Loughgiel fought back and a Shay Casey goal helped them to a two-point lead at the interval.
Once again though, it was the Galway men who were quicker out of the blocks from the restart, and despite Benny McCarry netting for the Shamrocks' second three-pointer, Connor Cooney's late free took the game to extra time.
St Thomas' took control in the first period of extra time with Gerry Murray's goal giving them a three point lead, and two further scores from the hugely impressive Cooney - who finished with a personal tally of 0-12 - helped his side to a three point lead with seconds left to play.
Watson was denied a goal in the dying moments when his effort was deflected over the bar, but the Shamrocks Talisman made no mistake with a fiercely-struck 21-yard free right at the death which beat no less than nine St Thomas' players on its way into the net.
That goal heralded the final whistle to bring the curtain down on what was a magnificent game, and ensured that the sides will meet again to decide who goes on to face Kilcormac-Killoughey of Offaly in the decider on St Patrick's Day.