Ryanair has scrapped 1,000 flights in August and September.
The airline has cut flights between the UK and the Republic of Ireland, because of Ireland's quarantine policy.
The Republic of Ireland requires all travellers except those from Northern Ireland to self-isolate for 14 days on arrival.
Because of a drop in demand for flights Ryanair have drastically cut the amount of flights.
It will result in the loss of more than 200,000 passengers travelling to and from Cork, Shannon, Knock and Kerry, the low-cost carrier said.
Ryanair said in a statement: "Last week when the UK and Northern Ireland removed travel restrictions on short haul flights to/from the European Union, Ireland became the only country in the EU with a blanket 14-day quarantine restriction on all arrivals from EU countries, most of which have lower Covid case rates than Ireland.
"It makes no sense, when governments all over Europe have opened up EU flights since 1st June and removed travel restrictions on intra-EU travel, that the Irish Government continues to treat countries like Germany, Denmark and Greece as if they were suffering similar levels of Covid as the USA, Brazil and India.
"Air travel between Ireland and the UK is being badly damaged by this ineffective 14-day quarantine.
"Ryanair will significantly reduce its flights between Ireland and the UK in Aug and Sept, to reflect this suppressed demand."
