Opener Andy Balbirnie’s superb 77 helped Ireland reach their victory target of 183 for five with one ball to spare.
It was Ireland’s second-ever win against Pakistan – their first since the 2007 World Cup – who had earlier posted 182 for six, with Babar Azam (57) and Saim Ayub (45) top-scoring.
Balbirnie struck two sixes and 10 fours in his 55-ball knock and he was given valuable support by Harry Tector (36 off 27 deliveries) and George Dockrell (24 off 12).
Gareth Delany (10 not out) and Curtis Campher (15 not out) came together for Ireland’s last partnership with their side on 167 for five, needing 17 off eight balls, and they scampered home for the winning run off a leg-bye.
The two sides will meet again at the same Castle Avenue venue in Clontarf on Sunday and next Tuesday.