Rebecca Stokell scored a half century as Merrion beat YMCA, while Laura Delany was in the runs against as defending champions Leinster edged out Malahide in a tight contest.
Stokell hit 7 fours and cleared the ropes once as she top scored with 64 from just 61 balls as Merrion posted 158 for 2.
She shared an opening stand of 138 in 17.3 overs with fellow international Leah Paul who struck six boundaries in her run-a-ball 52.
Any chance YMCA had of chasing down the challenging total disappeared from the first ball of the reply as Stokell caught Gaby Lewis off the bowling of Celeste Raack (2-4).
Stokell claimed 3 for 14 as YM struggled, only gaining respectability through Kate McEvoy's unbeaten 33 as they were all out for 96.
In the other match to beat the weather, defending champions Leinster edged out Malahide in a closely fought encounter at Observatory Lane.
(Leinster celebrate a wicket: Pic: Rodney Smythe)
Skipper Laura Delany scored an unbeaten 54 from 53 balls, sharing a run-a-ball stand of 68 with Rebecca Rolfe (34) for the second wicket, but it was her stand with Sophie MacMahon which was to prove decisive.
MacMahon struck four boundaries in a breezy 35 from just 23 balls, as she and Delany added 63 in just 39 balls to take Leinster to 138 for 2.
It didn't look to be enough as an opening stand of exactly 100 between skipper Caoimhe McCann and Amy Hunter looked to be taking unbeaten Malahide to another win.
However the stumping of Hunter (31) opened the door and Leinster managed to keep the runs down in the closing stages despite McCann's excellent 62 from 64 balls (4 fours).
In the end Leinster won by 12 runs to put themselves in pole position to clinch another trophy.