Credit card entering car park, however barriers were already up on exit.

It's possible that your registration plate was scanned when you entered so they will link your credit card to the reg. no. and as soon as you exit, your card will be charged. But I've never swiped a credit card to enter a car park so have no idea what will happen. Neither will anyone else here, unless you care to name the airport and someone might have an idea how their car park works.

This is how it works ...

To avoid queuing for car park paystations in the airport terminal you can now pay for car parking at the airport barrier by using your credit card on entry to the car park. To pay by credit card follow the below simple instructions. Insert your Credit/Debit card at the machine at the Entry barrier and the same card in the machine at the Exit barrier when leaving. Your card will be charged automatically at the published rates for the duration of your stay. Please note you must use the same card at both Entry and Exit barriers.

https://www.irelandwestairport.com/car_parking/drive-up-parking-rates

If there was a problem with the card reader or the data link to the payment processing company, they'd have no choice but to raise the barrier. I doubt if they will bill you. ANPR would have allowed them to scan the registrations of the exiting cars, that data could have been later uploaded and the linked credit cards billed. But it's not beyond the bounds of possibility that someone will view footage from a CCTV camera and do that manually.