Laura Delany hit a half century as Leinster edged out Merrion by 8 runs, while Kim Garth made 47 as Pembroke beat YMCA by five wickets despite an undefeated 84 by Kerrie Tomlinson.
The Irish skipper hit 6 fours in an unbeaten 52, sharing a fourth wicket stand of 73 with Ellen Ridgeway (30).
Sophie MacMahon also chipped in with 23 as the home side posted 137 for 4, with leg-spinner Celeste Raack (1-16) the best of the attack.
Naomi Matthews got the reply off to a brisk start hitting six boundaries in her 39, but despite 47 from Leah Paul, Merrion fell eight short on 129 for 6.
Jenny Sparrow (2-11) and Ava Canning both bowled tightly in the win.
At Claremont Road, Kerrie Tomlinson hit 12 fours and a six in her 84 not out from 67 balls to dominate the YMCA total of 121 for 9 - Robyn Lewis (11) was next highest scorer.
For visitors Pembroke Cliona Tucker picked up 3 for 19, while Hannah Little (2-17) and Kim Garth (1-11) did well.
Garth then top scored with 47 as she and fellow Irish international Shauna Kavanagh (36) added 83 for the second wicket to put them on the way to a five-wicket win with three overs to spare.
At Claremont Road, Pembroke beat YMCA by five wickets
YMCA 121/9 (20 overs; K Tomlinson 84*; C Tucker 3-19, H Little 2-17)
Pembroke 122/5 (17 overs; K Garth 47, S Kavanagh 36)
At Observatory Lane Rathmines, Leinster beat Merrion by 8 runs
Leinster 137/4 (20 overs; L Delany 52*, E Ridgeway 30, S MacMahon 23; C Raack 1-16)
Merrion 129/6 (20 overs; L Paul 47, N Matthews 39; J Sparrow 2-11, A Canning 2-20)