Ireland and Scotland played back-to-back matches at Oakhill on Thursday with the Scots winning the 50-over clash and the Irish coming out on top in the T20 contest.
The morning 50-over game saw the Irish slump to 28 for 6 with left-arm spinner Charlie Peet (4-13) and Lyle Robertson (3-19) doing the damage.
Seamus Lynch (34) and Mitchell Thompson (21) attempted a salvage operation with a 7th wicket stand of 53, but a final total of 103 was always going to be way short of par.
Tomas Mackintosh - who last month scored a tournament winning century against Ireland in the U19 World Cup Qualifiers - was again the Irish nemesis as he top scored with 41 as the Scots won by five wickets in the 30th over.
Mitchell Thompson claimed 2 for 31 , while there was one wicket apiece for Cillian McDonnell, Jamie Forbes and Theo Dempsey.
With the early finish and the weather prospects bleak for Friday, it was agreed to play an extra T20 contest given the beautiful afternoon.
Ollie Metcalfe (27) and Nathan McGuire (23) raced out of the blocks with an opening stand of 47 in four overs - both hitting five boundaries.
Jack Jarvis (3-21) and two wickets each for Peet and Roberston pegged the Irish back to 82 for 6, before James Hunter (30) and Archie Johnston (16) added 40 for the 7th wicket to help their side to 133.
McGuire and Cillian McDonnell then took three wickets apiece as Scotland were skittled for just 79.
The win gives the Irish the advantage in the Celtic Cup series. leading 2-1 with Friday's T20 to come.