The carrier said more than 200,000 passengers would be lost from Cork, Shannon, Knock and Kerry.
The airline said the “defective quarantine restriction on EU visitors” was affecting sales just as the UK and Northern Ireland opened up air bridges to most EU countries.
Ireland accounts for less than 8% of Ryanair’s traffic, but the carrier argues tourism and connectivity for Irish workers commuting to and from the UK “is now suffering unrecoverable losses”, as arriving EU passengers are forced to quarantine, even though the border to Northern Ireland remains open with no restrictions.