Ireland enjoyed the majority of territory and possession but they were frustrated for long periods as they kept running in to the brick wall that was America's defence.
Some poor decision making and over-eagerness also hampered Ireland before they were able to find their rhythm after the break.
Tommy Bowe (2) and Rory Best crossed for tries while Jonathan Sexton weighed in five points and Ronan O'Gara added a conversion.
James Paterson slotted over a penalty and converted Paul Emerick's interception try, while the US were massively impressive in defence with some big hits rocking Ireland's runners back.
Ireland had failed to take advantage of their early dominance as the USA showed determined resistance.
The Irish were all over the America pack in the scrums, while Sexton, Keith Earls, Jamie Heaslip and Conor Murray were particularly lively.
However they were guilty of forcing the pass too often and taking the wrong options as they struggled to find a way through the US defence.
Sexton missed two penalties before he finally slotted over to give Ireland the lead but it was scant reward for the efforts of Declan Kidney's side.
The US had a chance to level but Paterson saw his penalty effort go wide, Taku Ngwenya was asking plenty of questions of Ireland's defence.
However Bowe finally found a way through with 20 seconds of the half to go - with the winger escaping the clutches of Mike MacDonald before accelerating away to score. Sexton converted.
Sexton had a chance to extend Ireland's advantage three minutes after the restart but he was off-target for a third time - with a fourth miss quickly following.
That saw the introduction of O'Gara and Eoin Reddan at half-back as Ireland looked to switch tactics.
However they could do little to prevent the USA enjoy their best period as they drove and drove in to the Irish defence - before Paterson slotted over a penalty.
The Ireland response was immediate as Best powered off the back of a maul and through the defence for a try. O'Gara missed the conversion.
A third try quickly followed as Brian O'Driscoll scythed through before shipping the ball on for Bowe to provide the finish. O'Gara converted.
However, Ireland couldn't find a way through for a bonus point try - and it was the Americans who celebrated in the final minute as Emerick scored from an interception.
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