Ryanair is being investigated in Britain to see whether it broke consumer law there by failing to offer refunds for flights that passengers could not legally take because of coronavirus restrictions.
The UK's Competition and Markets Authority (CMA) has launched the enforcement action against both Ryanair and British Airways.
It has written to the airlines over its concern that, by failing to offer people their money back, they may have breached consumer law and left people "unfairly out of pocket".
"It is now seeking to resolve these concerns with the companies, which may include seeking refunds, or other redress, for affected customers", it added.
During periods of lockdown across the UK, Ryanair provided the option to re-book but refused to give refunds, it said.
Consumer journalist Siobhan Maguire told Newstalk Breakfast it is an interesting case.
"Anyone who had booked flights for 2020 would have been familiar with the fact that we were being told here in Ireland against non-essential travel.
"And yet would have seen flights taking off and still going to their destinations even if they were half empty.
"What you now see in the UK is the Competition and Markets Authority, the CMA, basically looking into whether airlines... essentially broke consumer law because consumers were lawfully unable to fly.
"Instead they were being offered re-routing options from Ryanair or re-routing or vouchers from BA."
She said the rules were muddied owing to the circumstances.
"The consumer laws were quite murky in relation to this area - because we're very clear on what happens if a flight is cancelled.
"But what happens if the flight still proceeds, but you're being told by your Government that you cannot fly - and that's exactly what happened here.
"They're saying that it was unfair - that people should have been fully refunded for something that they simply couldn't partake in".