The USA, with their forwards in fine form, dominated possession over the opening 40 minutes with a 65% share.
Three penalty goals from Mike Hercus - the last of which closed the gap to 12-9 early in the second half - had the Americans in touch but a 54th-minute try from man-of-the-match Neil Best and a penalty on the hour from Jonathan Sexton gave the Irish some comfort at 20-9.
As the Americans tired, having played against England Saxons four days ago, Ireland 'A' began to open up and play to their potential.
After USA prop Mike MacDonald was sin-binned, the tries flowed in the final ten minutes. A number of replacements caught the eye, with busy scrum half Frank Murphy probably the pick.
The result was put beyond any doubt thanks to late tries from Murphy, Mark McCrea and Denis Hurley. Ian Keatley, another replacement to impress, converted all three.
For a blow-by-blow account of the game, click here.
For a full match report with team and scoring details, click here.
Watch brief highlights of the Ireland 'A' v USA Churchill Cup game here.
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