McCarthy has so far brought in Frank Nouble on an 18-month contract from Wolves as well as loanees David McGoldrick, Aaron Mclean and Patrick Kisnorbo - although McGoldrick comes with a view to a summer move.
The Irish boss is keen to do more business before January 31 but, with no deals on the immediate horizon, he accepts it may have to be via the loan market.
"We'd like to get players in before the window closes, permanent ones but we are not having much joy at the moment," he told the club's official website.
"We're looking at loans now more than signings and I know I said that I didn't want to do that but at least if it's window to window, they stay with us until the end of the season.
"There are players out of contract and available on a free. That looks a good gig until you find out what the players are earning at the moment.
"Then you have the fact that you have to take them on to the end of the season. That's fine if it's May 4 but it's not like that, you have to take them up to July 31 (end of contract).
"If you don't sign them after that, it means you are paying them for three months football and three months to sit on the beach. That's just not viable."