"And it cost about 2,000 euros each, including our rail passes."
The Irish fans ensured a superb atmosphere in Niigata as the Republic drew 1-1 with Cameroon.
McCarthy's Ireland now play Germany on Wednesday night and a good result could help them on their way to the last 16 stage and a game in Korea.
The supporters are hoping the team does progress, even though many of them will not be able to go with them.
"I do not know what would happen. Many of us have not got the time off work to stay out that long," added the 23-year-old computer programmer who has his own World Cup diary at
"But we will just have to cross that bridge when we come to it."
Ailbhe and Grainne Burke were able to take advantage of the fact their sister had just started working in Japan.
"She took up a teaching post six weeks ago which was well-timed," said Ailbhe.
"We have been staying with her and our return flights from Dublin were just 900 euros, probably because we came out early, a week before the World Cup started.
"We flew out the day we heard Roy Keane was coming home and good riddance to him I say, because we are doing all right without him."
Tom Monaghan and his five friends from Cork found the trip quite expensive but felt it was money well spent.
"We have paid 4,000 euros each but we had no hesitation in coming here," said Tom.
"To get to this part of the world and see the culture is a one-off."
Also in Chiba, where the Irish team is based, was Jack Byrne, former manager of Dalkey United, the club where Paul McGrath started off playing.
"The support was brilliant in Cameroon," said Jack.
"There were two guys in front of me in Glasgow Rangers shirts.
"They said, as Scotland had not qualified, they had come to support us. I thought that was brilliant.
"I am due to go home on 17 June but if Ireland get through I hope to go to Korea if I can get my airline ticket changed."