The 74-year-old Italian had indicated earlier this week that 33-year-old stalwart Dunne would return to the heart of his defence for Friday night's World Cup qualifier against Sweden for the first time in a competitive game since the Euro 2012 finals.
But he sprang a surprise when he named his starting XI and included Wigan winger McClean in it as the injured Aiden McGeady's replacement.
Anthony Pilkington had been tipped to make a debut on the left, while striker Simon Cox was considered as an option, but it was 24-year-old McClean, discarded by Sunderland over the summer, who eventually got the nod after impressing in the corresponding fixture in Stockholm, a 0-0 draw in March.
Trapattoni said: "He played well, he is also physically strong and this game is a tough game - it will be a technical, but also a physical game against Sweden. They are strong physically.
"At first, I thought about playing with four strikers, but then I thought three strikers is already enough, and because also during the game the other options could be very important.
"During the game, I can also choose to have other tactical opportunities. That is the reason."
Dunne's return will add experience and know-how to a defence which has repeatedly laid the foundation for the Republic's better results under Trapattoni.
The manager still needed assurances over the decision to select a man who because of injury played only 25 minutes of football last season - and that came in June.
Discussing Dunne, now at QPR, Trapattoni said: "I said at the last conference, I have to think very well about Richard because firstly, it was a long injury and secondly, he was without a club.
"I said also I would follow him because his fitness would be important, and I have seen him very well with the same personality.
"I have seen him play very well in games and for me, he is the same Richard as he was before the injury."
Skipper Robbie Keane, who has spent much of his international career at the opposite end of the pitch to his fellow Dubliner, was delighted to see him back in harness.
Keane said: "He's a massive presence. He's my mate and I know how frustrating it has been for him for such a long time, to be out since the Euros, basically.
"I know how excited he is now to be back in the fold and desperate to play in a competitive game.
"He's had a good start to his new team with QPR, so he feels fit and he feels sharp. He's a massive, massive player for us.
"He has a massive presence when he plays for us and any player who comes up against Richard Dunne knows that they are in for a tough night."
Sweden's visit to the Aviva Stadium and the trip to Austria which follows four days later will go a long way towards deciding the destiny of Ireland's qualification hopes, and perhaps Trapattoni's continued presence in his job.
However, it is a challenge the veteran manager is relishing.
He said: "We are very conscious that in the next two games against Sweden and Austria, we are playing for our participation in the World Cup.
"I have watched the [last] match against Sweden and I am convinced that we have a good chance to win this game, because in Stockholm we played very well.
"Now we need to repeat the same performance with full commitment. Only with this can we think about achieving qualification, because after the game against Austria we can have greater morale."