UP TO 350 Dublin cellars will be filled in during the first phase of the €368m Luas Cross City project.
Some 1,500m of slit trenches will be dug to expose the cellars, many of them long-abandoned.
Clondalkin-based KN Network Services has been awarded the first contract in the scheme, which will join up the existing Red and Green Luas lines and link with the Maynooth/Dunboyne commuter line at Broombridge in Cabra.
The Department of Transport says it is necessary to fill in the cellars so that the next phases of the works can begin.
The contract will deal with 25 'known structures'; a further 330 cellars are expected to be identified and filled in as part of the first phase.
"Work has already started on the building condition surveys to assess the condition of properties along the Luas Cross City Line," it said.
Over the next three months, work will start on checking for cellars and once identified, they will be filled in.
Luas Cross City said: "Many city streets have cellars extending under the footpaths and the roadway. Many have been closed off by owners and in some cases filled in.
"While 25 have been identified along the route of the Luas Cross City, it is likely that further unknown cellars exist."
The Cellar Infill Contract starts this month.
The streets affected are: Dawson Street, Grafton Street Lower, Westmoreland Street, Marlborough Street, Parnell Street and Dominick Street.
"When a cellar is discovered under a street, a small hole is opened in the roof of the cellar and a camera inserted to survey the inside of the cellar and its condition," a Luas Cross City spokeswoman said.
"If it's safe to do so, a larger hole is then excavated to allow personnel to access the cellar and archaeologically record the cellar prior to its infill."
Luas Cross City says work sites related to the infilling will be relatively short, about 30m long.
"In most cases, pedestrian movement along footpaths will be maintained when cellars are being filled.
"However, pedestrians may have to be routed around footpath work areas."
Access for deliveries and to premises will be maintained during the course of the works.
The main construction works for the Luas Cross City project will start in early 2015.
The project is expected to create around 800 jobs during the construction phase.
Luas Cross City is expected to attract an extra 10m passenger journeys a year, producing up to 40m passenger journeys across the Luas network.