Online check in is mandatory with Ryanair. You have to print out a boarding pass or download it via Ryanair's check in app. This means that any passengers (who are not upgraded to Business plus) and who fail to check in online with Ryanair will face fees at the Airport.
Airport check in fees currently stand at (03/02/2019) £55 per person Airport check in per flight.
Airport boarding pass issue is £25 per Ryanair boarding pass.