Carli Lloyd slipped a short pass to Kai, who collected with her back to goal, then spun to cut a bouncing left-footed shot past Ireland goalkeeper Emma Byrne and into the lower right corner for her 15th goal of the season.
U.S. goalkeeper Briana Scurry preserved the win with an excellent save on Michele O'Brien in the 89th minute.
The second game of the 10-game Achieve Your Gold Tour drew 4,227 fans -- one of the smallest crowds ever to watch a soccer game at Giants Stadium.
The U.S. women (28-1-1 in 2008) complete the three-game series against the Irish on Saturday at Toyota Park in Bridgeview, Ill.
Sept. 17 in East Rutherford, N.J.
USA 1 Ireland 0. Goal: Kai 72.
USA -- Solo (Scurry, 46), Chalupny (White, 70), Rampone, Markgraf (Buehler, 46), Mitts, Boxx, Wagner, Lloyd, O'Reilly (Cox, 46), Kai, Hucles.
Ireland -- Byrne, Tracy, Fahey, Moran, McCormack, Grant, Hughes (Moylan, 82), McDonnell, O'Gorman, O'Brien, Curtis.
Referee: Sandra Serafini (USA).