The Real Madrid forward played 66 minutes of his country's friendly victory at New Jersey's MetLife Stadium.
Ronaldo, who had missed the previous two warm-up games with tendinitis in his left knee and a thigh problem, did not find his way on to the scoresheet, but showed flashes of his brilliance, setting up one of striker Hugo Almeida's two goals and rattling the foot of the post with a first-half free-kick.
Portugal boss Paulo Bento said: "Yes, it was important that he played and we would not hide it. A player like Ronaldo is important for any team, and that's the case for us. It's good that he is back."
"I think he had a good match alongside what we did collectively. Considering he has been without competitive football for a few weeks, I think he did well."
Richard Keogh, Vierinha and Fabio Coentrão also got on the scoresheet for the Portuguese, with James McClean claiming Ireland's solitary goal.