People can't travel on an intercity train this Christmas without booking in advance - according to Irish Rail.
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.
Irish Rail is asking customers to only travel if their journey is necessary and to avoid peak and busier times where possible.