Four new platforms are to be built at Dublin's Heuston station, costing €117 million. Funding will come from the Government and the European Union.
Remodelling track work to increase commuter and intercity services is already underway and a spokesman said this was going to lead to disruption.
The first impact will be on St Patrick's Day when the station will be closed from 9am until Monday afternoon. A bus service between the city and Newbridge will be in place.
There will be two further weekend closures: one later in the year and one in 2003. The improvements to Heuston coincide with improvements to the Kildare line, the first phase of which is due to be completed in June of next year.