Customers of Toolin Travel, which ceased trading yesterday, have been advised to contact the relevant airline, ferry company or travel agent to check the status of their booking.
They can then make alternative travel arrangements if necessary.
Toolin Travel, which has eight branches in Dublin and Co Kildare, has said none of its customers currently abroad will be stranded.
The company also said that people who have booked to travel in the immediate future will not be affected.
The Commission for Aviation Regulation has said it is not yet clear how many holidays booked for later in the summer will be cancelled but people will be able to claim a refund.
The Commission says it will have more information by Monday, and customers should check its website www.aviationreg.ie.
