Ferry fares for residents of the largest of the Aran Islands have gone up by almost 90%.
The cost of a return journey between Inis Mór and Galway is to go up €8 to €15 from today.
The increase comes after the existing operator, Island Ferries, sought a hike in the subvention it gets from the State for the service.
However, the Department of Arts, Heritage and the Gaeltacht said it could not sanction such a rise.
Island Ferries founder Paddy O'Brien says increased fuel costs mean he has no option but to increase fares.
The company will continue to run PSO services to Inis Oírr and Inis Meáin.
The cost of a return journey to the mainland for inhabitants of those islands is increasing from €8 to €10.
The Department says efforts have been made to negotiate an agreed solution but that it could not provide a 300% increase in the annual subvention to provide a service to Inis Mór.
Inis Mór co-operative manager Cathy Ní Ghoill says it is unacceptable that the State's most populated island no longer has a guaranteed ferry service.
She said islanders are worried that the service could be withdrawn in future if it was deemed to be not commercially viable.
Fianna Fáil's Agriculture spokesman called on the Department to bulk buy ferry tickets to Inis Mór and sell them at a lower price to islanders.
Éamon Ó Cuív says this is a huge increase for residents to have to absorb and he is proposing an emergency solution, where the Department would buy tickets and sell them at a discount to people living on the island.
Estimates range as to how many islanders would travel to and from the mainland on the two return services each day, with figures varying from 30 to 60 passengers.
Traffic is greater around weekends.