Disciplined defending from Ireland thwarted the early Portuguese attacks until the 17th minute, when a Ronaldo strike clattered the post from 20 yards before Bernardo Silva somehow sent the rebound wide with the goal gaping.
A rare sustained period of Irish pressure in the 25th minute resulted in Jayson Molumby firing over from range, and though Portugal quickly regained momentum, a Pedro Neto effort dragged wide was the best they could create.
Late in the half, Caoimhín Kelleher was forced to backtrack when a Vitinha strike took a wicked deflection and landed on the roof of his net, before the visiting goalkeeper made a smart reaction stop to deny a near-post header from Gonçalo Inácio and did well to get behind a Bruno Fernandes hit.
The Boys in Green needed a vital block from Nathan Collins to deny Silva in added time, but they got to HT without conceding for the first time in this qualification campaign.
The break did nothing to halt the UEFA Nations League winners’ dominance, as Ronaldo flashed a brace of chances wide of the post in the first 10 minutes.
And just before the hour mark, space opened up for Rúben Neves to have a go, but Kelleher got down well to the curling effort.
With 20 minutes to play, Nélson Semedo then pulled a dangerous ball across goal, but Ronaldo was once again off target with his first-time shot.
Irish resistance looked to have finally been broken as the match entered the final 15 minutes, when Dara O’Shea rushed out to block a shot, and despite claims that he had used his chest, referee Ivan Kružliak awarded Portugal a penalty for handball.
Ronaldo stepped up, but Kelleher denied him with an excellent stop with his foot.
Roberto Martínez’s men threw everything at Ireland late on, and finally found a way through in stoppage time when a superb Francisco Trincão cross was headed home by Neves for his first international goal.
Portugal move on to six wins in a row, while Ireland remain on just one point in the group and without a H2H win for 20 years.
Flashscore Man of the Match: Rúben Neves (Portugal)
