GLEN Hansard, star of Fox Searchlight’s smash indie “Once,” has robbed the cradle, and he couldn’t be happier. In the movie, the Irish rocker, 37, and his co-star, Markéta Irglová, 19, play musicians who experience sexual tension although they ultimately remain platonic. But on their recent, cross-country bus tour to promote the flick, nature took its course and Hansard and Irglová “fell for each other and started a real-life romance,” a spy says. “But Markéta is still in high school back in the Czech Republic, so she had to rush home to finish her school exams.”