The Real Madrid forward, who missed his country's previous two warm-up games because of thigh and knee injuries, had hand in a goal and hit a post on his return as Portugal concluded their preparations for Brazil.
Captain Ronaldo was substituted on 65 minutes and was replaced by club-mate Pepe, another whose preparations for the South American showpiece had been badly hindered by injury.
Head coach Paulo Bento said: "Ronaldo played a good match, especially when considering that he has not played competitively for two weeks.
"Players like him are important for any team, and that is the case with ourselves too. It's good to have him back."
Ronaldo took 51 seconds to show that he was back firing on all cylinders, picking the ball up and powering towards goal, but his shot from outside the box was saved by Irish keeper David Forde.
His side did take the lead with their next attack after three minutes, however, as Silvestre Varela crossed from the right and striker Hugo Almeida rose unmarked to head home.
Sergio Ramos out for revenge in Brazil
The talisman Ronaldo was inches away from grabbing a second on 19 minutes as his free-kick bounced back off the post – but it was 2-0 a minute later as a Fabio Coentrao cross deflected off the Irish boot of Richard Keogh and into the net.
Ronaldo had a hand in the third goal on 37 minutes. His header was saved but Almeida was quickest to react as he netted his second of the match.
James McClean pulled a goal back for Ireland, but Portugal added gloss to the scoreline in the closing stages as Nani, a substitute, set up Vieirinha to thrash home and then set-up Coentrao for number five.
Portugal, who flew to Brazil immediately after the match, start their World Cup against Germany on 16 June in Group G - which also includes the USA and Ghana.
Despite being reigning world champions, Spain have revenge on their mind according to experienced defender Sergio Ramos.
Although they won the tournament in South Africa four years ago, they suffered a 3-0 defeat to hosts Brazil last summer in the Confederations Cup final. And now Ramos says that he and his team-mates have the ideal opportunity to get their own back.
"We've already been in Brazil for the Confederations Cup -- football always gives you a chance for revenge after you get this little thorn in your side," said the defender.
"We're the champions, there's high expectation. It's always possible but winning a World Cup is very difficult."
