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. (Agencies)
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.
The Portuguese will expect to make it to the knock-out rounds, but Vieirinha cautioned: "We all know that the first game is always the most important.
"We know how good the Germans are, but we also know what we can do against them. There are a lot of good teams at the World Cup,” he said.
"But we just need to do our work and what we do on the pitch is our responsibility, nobody else's, he added.
"Our first goal is to get through the group stage, and if we do that, we'll see how far we can go," said Vieirinha.
Ronaldo trained with the full squad again on Sunday at the New York Jets' training center, as did experienced midfielder Raul Meireles, another who has been sidelined with injury.
Portugal's remaining injury worry is feisty Real Madrid defender Pepe, who has been on a separate rehabilitation program as he returns after a calf injury.
The centre back trained Sunday away from the rest of the squad and is unlikely to feature against the Irish.
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.