The 18-year-old, who was picked up by the Delhi Capitals in the inaugural Women's Premier League auction while in the field for England on Monday, set up her side for victory with 34 balls to spare.
England had bowled Ireland out for just 105, but there were some warning signs for Heather Knight's side as they reached the total with ease, but lost six wickets in the process.
Capsey, who played for Melbourne Stars in the BBL, was caught by a diving Leah Paul at long-off, to finish with 51 from just 22 deliveries.
England had been cruising towards the win, at one for 70, but lost a number of quick wickets before Katherine Sciver-Brunt hit the winning runs.
Ireland came into the game full of confidence after a shock victory over Australia in the warm-ups ahead of the tournament, but were unable to capitalise on a quick-fire start with the bat.
Gaby Lewis top-scored for Ireland with 36 at the top of the order, but Ireland slid from 2 for 80 to 105 all out. Eight wickets fell to spin, three apiece for Sophie Ecclestone and Sarah Glenn and two for Charlie Dean.
England top Group B with two wins from two matches.
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