Portugal head coach Fernando Santos has said that his side struggled to deal with an "aggressive" Republic of Ireland side in their World Cup qualifier stalemate.

Portugal, who needed two last-gasp Cristiano Ronaldo goals to win the reverse fixture in Faro in September, were unable to break the deadlock at the Aviva on a night when clear-cut chances were at a premium at both ends in a match that ended scoreless.

Ronaldo passed up a glorious 67th-minute opportunity and was denied at the death by keeper Gavin Bazunu, who saved his penalty at the Estadio Algarve, on a frustrating evening for the visitors.

Portugal's cause was not helped by some less than effective defending, and the dismissal of 38-year-old stalwart Pepe for a second bookable offence eight minutes from time means he will be able to play no part in their final group game against Serbia.

Santos said: "Obviously, he wanted to win and not draw. We had a lot of difficulty and we were never able to turn on the game. There was a lot of difficulty in dealing with a very aggressive, very physical opponent," he told the Portuguese Football Federation's official website.

"In terms of defensive organisation, we allowed a lot of space. We had a hard time dealing with the game. They [Ireland] were always very fast and they diminished our ability to hold the ball. We could have done better."

Santos remains convinced Portugal will make it to next year's World Cup finals despite the result.

The former European champions will head into Sunday's final qualifier against Serbia in Lisbon sitting at the top of Group A, but only by virtue of goal difference after Thursday night's draw.

Win, lose or draw at the Aviva Stadium, Portugal were always going to need a positive result from their final match to guarantee automatic qualification, and head coach Santos insists the mindset has to remain unchanged.

"A draw or a 5-0 win was the same thing, but of course I would rather win. It's a positive result and we know that.

"At this moment, the Serbia team has to play in our stadium to win the game. I'm convinced we're going to be at the World Cup."

If Santos will be unable to select Pepe for the crunch clash with Serbia, he will at least be able to call upon all six of the men who went into the game at the Aviva stadium knowing they were one yellow card away from suspension.

He opted to leave five of them - Joao Cancelo, Diogo Jota, Renato Sanches, Ruben Dias and Jose Fonte - out of his starting line-up, and the sixth, Joao Palhinha, as well as second-half substitutes Fonte and Sanches, emerged unscathed.