The 20,000 tonne concrete structure was sunk beneath the river in a pre-dredged channel as part of the €500m Limerick tunnel project.
The tunnel will be 915m in length.
It is one of the largest infrastructural projects in the west of Ireland.
It will also complete the Limerick south ring road project, which will give Limerick its fourth river crossing, linking the city's dock road with the dual carriageway leading to Shannon and Ennis.
The four-year project is due to be completed in 2010.
