From the 18th of December to January 6th, everyone will have to reserve a seat, including passengers using free travel passes and season tickets.
The rule kicks in the same day travel between counties will be allowed - however doesn't apply to commuter or DART services.
Irish Rail spokesperson Barry Kenny says it's the only fair way to share out seats at half-capacity.
"First time in two months for most counties and three months for people in Dublin, that inter County, non essential travel has been permitted, and with 50% of capacity available for use on public transport including intercity rail. So we are advised people that intend to take the train during this period December 18th to January 6th, don't leave it to the day of travel and certainly don't expect to be able to purchase tickets in the station on the day."
