Ronaldo has missed Portugal's last two World Cup warm-up games against Greece and Mexico because of knee and thigh injuries that hampered his involvement in Real Madrid's end-of-season run-in.
But the 29-year-old, who helped Real to UEFA Champions League glory last month, returned to training on Friday and will be hopeful of gaining some match practice before the tournament gets under way on Thursday.
And his prospective comeback will come as a timely boost for Paulo Bento's men, who will look to build momentum ahead of a daunting World Cup opener with Germany a week on Monday.
However, Portugal may have to cope without midfielder Raul Meireles because of a muscular problem, while defender Pepe is continuing his recovery from a leg injury.
Ireland are winless in their last five matches under coach Martin O'Neill, but should head into the encounter at MetLife Stadium in New Jersey with confidence following decent results against World Cup participants.
O'Neill's men held four-time winners Italy to a 0-0 draw late last month and claimed a 1-1 stalemate against Costa Rica on Saturday.
"It's going to be a balanced group with four strong teams so it's going to be very difficult for us," Bento said after the win over Mexico.
"But we will fight to the end to achieve our first goal, which is to reach the knockout phase.
"We still have a few mistakes that we need to correct before the World Cup."
