Iarnród Éireann is warning of “extremely high booking levels” for this weekend.
Irish Rail said most trains between Dublin and Galway, Westport and Belfast are already sold out.
Many services from Galway to Limerick are also sold out.
It warned that trains are also sold on other routes.
Public transport is still operating at 50% capacity due to COVID restrictions.
🚨 Another busy weekend ahead
❗️Capacity at 50%, Intercity pre-booking mandatory, including for free travel & valid tickets holders at https://t.co/r50m2431AZ
🚆 Many trains 2nd to 4th July SOLD OUT between Dublin & Galway, Belfast, Tralee, Cork, Waterford, Rosslare, Westport pic.twitter.com/NJ1m6lvnpA
— Iarnród Éireann #StaySafe (@IrishRail) July 1, 2021
Passengers are being reminded that they need to pre-book to travel on Intercity trains.
People holding free travel passes and other valid tickets still need to book their place on the train.