The 29-year-old forward has missed Portugal's previous two warm-up matches through injury, hindering their preparations for the Brazil tournament and sparking speculation that he might not be fit in time.
But the Real Madrid ace returned to training on Saturday with Paulo Bento's side, who have failed to sparkle in their captain's absence.
Portuguese officials were tight-lipped Sunday about Ronaldo's availability for the Ireland encounter, but forward Vieirinha told reporters: "If he is fit to train, he is fit to play the match."
Portugal face a daunting World Cup opener on June 16 against Germany in Group G, which also comprises the USA and Ghana.
Bento has been keen to stress that Portugal -- who only made it to Brazil via a play-off with Sweden that saw Ronaldo fire a hat-trick -- are more than just one player.
But Wolfsburg man Vieirinha admitted their star player's return had buoyed the whole squad.
"Everybody knows the importance of Ronaldo to our team and his importance to the group, both on and off the pitch," said Vieirinha, 28.
Ronaldo has scored 49 goals in 110 matches for Portugal.
Portugal have failed to impress without Ronaldo, who has been hampered by a left thigh muscle problem and tendinitis in his left knee.
Last weekend they labored to a 0-0 draw in Lisbon against fellow World Cup qualifiers Greece.
Then on Friday, near Boston, they survived several scares before beating Mexico - who will also be in Brazil - 1-0 thanks to an injury-time header from Bruno Alves.