AN influx of visitors from the Republic is expected at the weekend with trains from Dublin to Belfast having almost sold out.
Irish Rail said it had seen "very high demand" particularly on the Enterprise service to Belfast for the July 12 holidays.
Only 50 per cent of on-board capacity is available for use due to Covid-19 restrictions.
The arrival of southern visitors travelling this weekend is in complete contrast to the usual exodus of northern visitors travelling to the Republic during the annual Twelfth holidays.
But with some coronavirus restictions remaining in the Republic, including a ban on indoor dining, more visitors are travelling over the border.
Iarnród Éireann said many trains were already sold out for this weekend, particularly on the Dublin to Belfast and Dublin to Galway routes, with more trains expected to sell out on all services across the network.
A spokesperson for Iarnród Éireann said: "With more sectors of the economy open, and particularly with domestic tourism resumed, we expect many Intercity and Enterprise services to sell out in advance over the coming weeks, and particularly at weekends, with 50 per cent of on board capacity available for use.
"That is why we are stressing to those who wish to travel that it is essential to book in advance, as otherwise you will not be able to travel.
"Customers have been hugely cooperative and supportive throughout the past 16 months with all of the public health measures, and we are grateful for their further assistance with mandatory pre-booking."
