Staff absences at Irish Rail because of Covid-19 have resulted in a number of trains being cancelled.
Around 20 Phoenix Park Tunnel and Heuston Commuter services will not run between now and Friday.
Spokesperson Barry Kenny says they have been doing everything to keep trains going.
"Like every employer, every service provider at the moment, with the number of cases that are in the country it isn't something we can escape. Particularly in Heuston at the moment there are a number there [staff out with Covid] so unfortunately it is unavoidable."
Meanwhile, the HSE’s lead on the vaccination and contact tracing programmes Damien McCallion has acknowledged that the number of Omicron cases is “substantially higher” than the levels officially recorded through PCR tests.
Speaking on both Newstalk Breakfast and RTÉ radio’s Morning Ireland, Mr McCallion encouraged anyone with a positive antigen test to act as if they had the virus and to follow public health guidance.