Shahid Iqbal proved the star as Rush held their nerve to beat Railway Union to set up a play-off against Leinster, while Phoenix are relegated after a poor batting display at The Hills.
Former Irish international Kenny Carroll hit 9 fours and 2 sixes as he top scored with 69 as Railway posted 203 at Kenure.
Rachit Gaur made a surprise return to the Railway line-up - he has been playing for Strabane in the NW Premier league - but went cheaply.
Another new face in the Railway line-up was former Zimbabwean U19 World Cup player Glenn Querl - who is now married and living in Kildare.
Once on the books of Hampshire, he showed glimpses of his power, hitting two sixes in his 33, adding 55 for the fifth wicket with Carroll.
Shahid took 4 for 46, while Alex Neary picked up 3 for 35 and Daniel Coffey 2 for 27.
There were plenty of nerves around in a tense chase, with rain and the threat of DLS an ever increasing Damoclean Sword, but thankfully the game was completed despite the drizzle.
At 112 for 5 the outcome was in doubt, but Shahid showed all his experience as he guided his side over the line with an unbeaten half century from 48 balls (5 fours, 2 sixes).
He shared what was to prove a crucial 6th wicket stand of 73 with Subby Ramnathphur (47) as Rush prevailed by four wickets.
The Cork County match against Terenure fell victim to the weather, while Matt Ford's sixth hundred of the season saw Malahide complete a 100% record as they beat Balbriggan by 57 runs.
In the Premier Division, Merrion received the trophy on a day full of emotion following the sad death earlier this year of Damien Poder.
Dom Joyce (84), John Anderson (82) and Tyrone Kane (78) all scored half centuries as they posted 312 for 9 - all this despite Ropu Islam taking a hat-trick.
Bill Coghlan hit 111 in the reply, but Clontarf finished on 256.
At the bottom, Phoenix were relegated after managing just 96 at The Hills - Reinhardt Strydom scoring 33 of those.
Albert van der Merwe and Kyllin Vardhan made 28 apiece in the chase as the Hills won by six wickets.
Leinster went down in a thriller at home to North County, losing by three runs.
Eddie Richardson hit 11 fours and 4 sixes on his way to 103 out of County's total of 191 - three wickets for Gareth Delany (3-26) and two each for George Dockrell, Bilal Azhar and Joe Carroll.
Hugh MacDonnell hit 63 in the Leinster reply, but Adam Coughlan accounted for the big three of Patel, George Dockrell and Joe Carroll to stunt the chase. 16 year-old seamer Owen Graham's four wickets ensured County finished the season on a winning note.
The D4 bragging rights went to Pembroke as they thumped YMCA by 108 runs at Claremont Road.
Jack Balbirnie top scored with 65 as Pembroke made 247, and then took four wickets as they dismissed the hosts for 139.
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