Mullingar train station will benefit from the additional carriages.
There will be an increased capacity on a rail route serving a Westmeath town.
Selected trains travelling between Connolly Station in Dublin to Sligo will see two more carriages added to the journey that also includes Mullingar.
Six services, from between 5 past 9 in the morning to 7pm in the evening will have six carriages for passengers.
Irish Rail has taken in a delivery of 41 new carriages for its intercity network.