Martin O’Neill blamed a nervous start as Ireland’s winless streak stretched to six games with a 5-1 loss to Portugal.
Fit-again Ronaldo and his team-mates needed only two minutes to force their way in front in New Jersey, Hugo Almeida capitalising on an inch-perfect cross from impressive winger Varela to head firmly past keeper David Forde.
They doubled their lead with 20 minutes gone, although with the help of a slice of good fortune when Fabio Coentrao's cross clipped defender Richard Keogh and looped over Forde.
The Ireland keeper was beaten again eight minutes before the break when he could only block Ronaldo's header to Almeida, who duly obliged, and, although James McClean pulled one back for O'Neill's men seven minutes after the break with his first senior goal for his country, hope of a fightback was short-lived.
It was substitute Nani who re-imposed Portugal's superiority, first feeding fellow newcomer Vierinha to score from close range before setting up Coentrao to round things off seven minutes from time.
It proved a sobering evening for O'Neill and his players as their summer programme drew to a disappointing close.
The manager said: "I thought we started nervously, and that was something we wanted to try and avoid. Two of their goals came from our possession, which we gave away needlessly.
"Those are the things that over the course of the European championships you have to try and cut out. I thought we came to grips with it - we got the goal back and looked dangerous at times.
"We were still trying to chase it to get back to 3-2. When I finally sit down and analyse it, there should be things I am happy about and obviously things we need to improve."
"Two of their goals came from our possession, which we gave away needlessly"
O'Neill was not too concerned at the margin of defeat, insisting that, had his team shut up shop at 3-1, it might not have seemed too bad a result.
He said: "I feel it is not disheartening in the sense that it was 3-1 and if we shut up shop, it is 3-1 and it doesn't seem so bad.
"I am not too bothered. I have done this before at club level, trying to chase games and stay as positive as possible, and considering the changes that we made and some of our main players not playing, I don't think this will kill us off.
"It was as tough a game as we expected it to be. But it was also game that I wanted us to participate in because this it is what it is about: playing one of the best teams in the world heading to the World Cup.
"Portugal played well throughout the match and it was a stern test for us."
Meanwhile, Portugal boss Paulo Bento was a happy man after seeing Ronaldo safely negotiate his way through his return from injury ahead of the World Cup finals.
The Real Madrid forward played 66 minutes of the match.
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."
Ronaldo's contribution will put him in line for Portugal's opener against Germany in Salvador on 16 June, when Bento will hope he has regained a little more of his customary sharpness.
However, he was content enough with what he saw against the Republic.
The manager said: "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."