"There is a difference between a good football team and a team of well-known world stars," he said.
"We respect Spain as a good side, but we are not scared of anybody. I do not like the word fear.
"Anybody who suggests we cannot beat Spain must be crazy.
"Our results and performances have been just too good for that.
"It is complete nonsense if anybody thinks that.
"I have seen it too often against Irish teams, very confident sides going away with a bit of a spanking."
McCarthy's mood was not helped when the first questions asked suggested he had complained about Ireland's training facilities in Seoul.
"The facilities are excellent," he said.
"We had a great result the other night, qualified for the last 16 of the World Cup and I am here talking about bull...again.
"I have not made a complaint to Fifa."
Several key players did not take part in Thursday's training session because of varying niggling injuries.
But Damien Duff (knee), Steve Staunton (thigh), Robbie Keane (groin) and Kevin Kilbane (ankle) are all expected to be available for the game against Spain.
Although Ireland coped admirably with the weight of expectation prior to the Saudi Arabia game, McCarthy is content to let Spain take over the mantle of favourites.
"We can be more relaxed, suddenly the pressure is not the same.
"It is a horrible thing in football, being expected to beat a team.
"It is a difficult pressure to deal with."