Gaby Lewis top-scored with 39 from 33 balls in an Ireland total of 145 for six at Sydney Parade on Sunday before Sri Lanka eased to a seven-wicket win in the first of back-to-back T20 internationals.
Put in to bat, Amy Hunter got Ireland off to a rapid start with 17 from 12 balls, then Lewis and Orla Pendergast built a solid platform to the innings with a second-wicket partnership of 59.
In-form Pendergast was stumped for 29 from 27, and when Lewis fell in the following 13th over, after striking five boundaries, the Girls in Green managed only 51 runs from their last eight overs.
In reply, Sri Lanka raced to 50 not out at the end of their six-over powerplay, and while Freya Sargeant, Laura Delany and Arlene Kelly each took a wicket, the visitors were rarely troubled and won with 20 balls to spare.
Opener Harshitha Samarawickrama, who sealed the victory with three successive fours off Prendergast to end with 86 not out from 45 balls, was Player of the Match.
The two sides will meet again at the Pembroke club on Tuesday before a three-match one-day international series in Belfast.