Kyle McCallan scored ten off the last two balls to propel Waringstown into another final and break CIYMS hearts, while Pembroke will be their opponents after a routine seven wickets victory against Phoenix.
It all looked lost for Waringstown, who needing 10 from the last over bowled by Obus Pienaar only managed two singles from the first four.
However, cometh the hour cometh the man, and the wily McCallan first smashed a six off the penultimate ball, and then with two needed scored a four to spark scenes of wild celebration in another thriller between these two titans.
CIYMS had picked up two trophies in recent weeks but the Irish Senior Cup was the one trophy they cherish more than most, and they go another season coming up short.
For Waringstown, they will have a chance to further extend their record as the competition's most successful side to seven in the decider at The Vineyard (The Hills) on August 31st.
Chasing 207 they were given a solid platform by Adam Dennison's 50, with James McCollum (33) and Lee Nelson (20) all chipping in.
They were creeping behind the rate when McCallan joined overseas player Haseeb Azam in the middle with 65 needed off the last 10 overs.
The much maligned Azam scored a valuable 40 not out, while McCallan will never have made a more vital 24.
Earlier Azam had conceded just 7 runs in his 10 overs as CIYMS found the going tough.
Chris Dougherty (52) and Jason van der Merwe (47) dug in, and with late runs from Jacob Mulder (22), James Cameron-Dow (16*) and Trevor Britton (11*) a total of 206 for 8 gave them something to bowl at.
In the end it was nearly enough, but not quite!
It was all much more routine at Phoenix Park were Phoenix were steam-rollered by Pembroke.
Barry McCarthy's new-ball burst accounted for the pivotal wickets of Simi Singh and Ben Beecroft - both without scoring - on his way to 4 for 13 as Phoenix slipped to 16 for 4.
They recovered with Nic Pretorius (48) and Poonish Methta (38) adding 74 in 16 overs, before Jack Balbirnie (2-37) broke the stand.
Shaheen Khan then ripped through the lower order taking 4 for 14 as Phoenix were dismissed for 136.
That modest target proved a stroll in the park despite the early loss of Jack Balbirnie.
Skipper Fiachra Tucker made 29, then the Irish duo of Andy Balbirnie and Lorcan Tucker took centre stage.
Balbirnie hit 6 fours and a six in his 54 from just 46 balls, sharing a third wicket stand of 61 in 13 overs with Lorcan Tucker, who cleared the ropes twice in an unbeaten 41 to seal a seven wickets win.
It should be quite a contest between two of the country's top sides at The Hills - roll on August 31st!