"Who knows if we are going to win this summer but we're going to go into it with the right attitude, the right environment, and the right team which believes it's got a chance," McCullum told British media on Monday.
"We're a team that knows how we want to play now, and we're not afraid to lose, which is quite a remarkable trait, and that's led by the skipper and some of the senior players as well.
"If we do play how we want to play, we give ourselves the best opportunity and we've got the talent to put world-class teams under pressure."
McCullum said fast bowlers James Anderson (groin) and Ollie Robinson (ankle) would miss the Ireland test due to injury but will be fit for the Ashes opener.
All-rounder Stokes, who had an injury-hit stint with Chennai Super Kings in the Indian Premier League, could play a role with the ball during the Ashes, he added.
"Stokesy is progressing well too. He looks really fit as well, looks in great order. I think he'll bowl at some stage throughout the summer, no doubt.
"He is a world-class all-rounder and if he is able to bowl, fantastic. If not, we'll find a way."