The centre-back headed home in the last minute as Ipswich recovered from going behind after just 34 seconds at Portman Road.
To cap a great night for the Suffolk side, four of their nearest rivals failed to win, keeping Ipswich well and truly in the hunt for Premier League football next season.
McCarthy said: "We will just keep ticking away and trying to pick up points. I'm very pleased with the result and the response after conceding a goal after less than a minute.
"It was more like us (after the break), as I don't think we laid a finger on them in the first half. But I thought we deserved the win in the second half."
Derby got off to a flyer thanks to Patrick Bamford's sixth goal in a productive loan spell, the Chelsea forward meeting Craig Forsyth's cross and sending the ball beyond Dean Gerken with little over half a minute played.
A dismal first half livened up after the break, largely thanks to a superlative 30-yard strike from another Premier League loanee - Jonny Williams successfully trying his luck from distance to grab an equaliser for the hosts.
Both sides could have grabbed a winner, Forsyth and Sylvan Ebanks-Blake both denied by the woodwork, before Berra rose highest to head home Aaron Cresswell's injury-time corner.
McCarthy added: "Jonny Williams was kicked from pillar to post in the first half but the best way to get your own back is to put the ball in the net.
"Christophe Berra has been outstanding all season, so it was fitting he scored the winner."
Fresh from their derby heroics, beating hapless Nottingham Forest 5-0 on Saturday, Steve McClaren's side were brought crashing down to earth.
Their unlikely pursuit of the top-two now appears over, but the Derby boss has called on his team to right some wrongs following the dramatic defeat in Suffolk.
McClaren said: "We have Charlton on Saturday and we have to take our disappointment and anger and make sure we don't let another one slip.
"I thought we controlled the game, created chances but there was not enough care in the final third.
"Our keeper has not had a shot to save apart from picking the ball out of the net from two set-pieces. We caused our own problems in the end.
"I can't fault the attitude and the football was great. We had a good opportunity, but in the end we paid the price."