Quarantines are ineffective because they cannot be policed, according to Ryanair DAC chief executive Eddie Wilson.
The UK plans to introduce a 14-day quarantine for incoming travellers, but also says it will exempt some countries, including the Republic.
Mr Wilson, who runs Ryanair Holdings' largest airline company, dismissed the plan as "unpoliceable" and "unimplementable" on Today with Sarah McInerney on RTÉ Radio 1.
“It sounds like a good idea, but when people arrive in airports, they end up going on public transport. And what do you do, do you take everybody who’s on the Gatwick Express and Stansted Express and isolate them as well?”
Ryanair intends to resume flying with 40 per cent of its schedule from July 1st, but with temperature checks at airports and precautions such as requiring passengers to wear masks on board.
The group is seeking to cut 3,250 jobs across flight crews and four offices. It also wants to reduce salaries by 20 per cent.
Mr Wilson said the airline would restore pay and jobs as growth in its business ultimately resumed.
“There will inevitably be job losses, because if you are going to fly 50 per cent less than you had hoped to do, that has to have an effect on the number of people that we have. We will return to growth and those jobs will come back at some stage.”