Ryanair plans to remove 48 services from Dublin airport at the start of next year.
The airline is cutting the Dublin-Cork route two daily return flights to one.
Services to East Midlands, Edinburgh, Glasgow, Leeds Bradford, and Manchester are also being scaled back.
Fine Gael said Ryanair's decision meant that consumers were losing travel options while businesses were suffering due to a decline in tourism.
"We need easy, cheap and regular access for tourists, for business travellers, for our growing diaspora and of course to transport fragile, high value exports," said the party's competition and consumer protection spokeswoman Olivia Mitchell.
Separately, around 90 angry passengers spent almost five hours on a Ryanair plane refusing to disembark after their flight was diverted to Liege instead of landing at Beauvais airport in northern France.
Passengers from three other diverted Ryanair planes accepted an offer of bus transport to France when fog shut down the Beauvais airport, 225 miles away.
But the mostly French tourists returning from Morocco on a fourth Ryanair plane refused to leave when it landed about midnight in Liege.
A spokesman for the Liege airport, said the situation was tense and some passengers “were very aggressive, very rude.” He said passengers finally left the plane at the request of police.
"It is standard safety procedure for all airlines to divert to another airport when weather closes a destination airport, and it is standard procedure to coach passengers to their destination," Ryanair said in a statement.