Australian Adam Ward put a serious dent in the YMCA title challenge as his Leinster side crushed the Claremont Road side by 124 runs at Rathmines.
The 20 year-old Victorian top scored with 96 from 89 balls (13 fours, 2 sixes) as Leinster scored 260 for 6. There were useful cameos from Craig Mallon (39), Richard Forrest (33) and Stephen Moreton (31).
YM, without Reinhardt Strydom and Trent Johnston, were never in the hunt, dismissed for 135 with only Jeremy Bray (44) and Jack Tector (23) offering any significant resistance.
The impressive Bilal Azhar (3-31) and Stephen Moreton (3-33) took the bowling honours.
Merrion took advantage of the slip up to go top of the table after a hard fought four wicket win at Park Avenue against Railway Union.
Half centuries from Paddy Conliffe (69) and Kenny Carroll (51) helped Railway to 217 - Tyrone Kane (4-42) the pick of the Anglesea Road side.
A solid run chase with 3 half century partnerships was enough for Merrion to get the vital win. Brett Thompson hit 9 fours in his top score of 62, while skipper Dom Joyce (35), Damien Poder (33), Peter Blakeney (24) and Dave Watkins (23*) all made crucial contributions.
The Hills won the bragging rights in the Fingal derby after posting 313 for 8 at North County.
Mike Baumgart's century from 127 balls (14 fours, 3 sixes) was the highlight, sharing a third wicket stand of 93 with Nic Pretorius (44).
That formed the perfect platform for fireworks later in the innings with Naseer Shaukat (57) and Max Sorensen (46) adding 83 in just 33 balls, with Eddie Richardson (3-73) and John Mooney (1-84) coming in for some serious punishment.
North County were never in contention despite the best efforts of Mooney, whose unbeaten 97 came from 95 balls (12 fours, 3 sixes). He was denied a deserved century by Max Sorensen bowling last man Ian Bertram, after the pair had added 64 - the best stand of the game.
Mark Dwyer (3-37) and Osama Khan (3-47) were the pick of The Hills attack as County were dismissed for 209 in the 41st over.
Phoenix's relegation from the top flight was confirmed after they slumped to an 113 run loss at Pembroke.
An opening stand of 166 between Theo Lawson (89) and Ryan Hopkins (73) - which contained 25 boundaries - helped Pembroke post 265 for 5, and their reply never got going, dismissed for 152.