Shane O'Neill was born in Ireland, but primarily raised in the United States. He is a citizen of both and, as a promising young defender, both countries' national teams want him. Jurgen Klinsmann doesn't seem to have any doubt that he will pick the U.S., though.
Jurgen asked if Shane O’Neill is in his plans. "Yes, he’s in our plans." When reminded O’Neill is Irish-born, JK "Yes, but he’ll go with us"— Ives Galarcep (@SoccerByIves) November 17, 2014
O'Neill was a regular for the U.S. U-20s -- where he first made the switch from midfield to defense, and has stayed ever since -- and played for the U-23s as well. He also was a part of the senior team's January camp, but did not make the cut for the friendly.
O'Neill has indicated that he might accept a call-up to Ireland so it is not as if his playing for the U.S. is a gimme. He clearly has some level of interest in the country of his birth, but has identified with the U.S. throughout his playing career this far.
There is no such thing as too much talent and, as the U.S. has learned, that is especially true in central defense. Nabbing a 21-year-old like O'Neill, who has been good for the Colorado Rapids and has the prototypical build for a central defender is huge.
There's reason to believe Klinsmann when he says he expects O'Neill to choose the U.S. For one, he has presumably spoken with O'Neill, but he also has had a great track record of getting dual nationals to pick the U.S. so O'Neill sticking with the Americans seems like a good bet.