The National Bus and Rail Worker's Union says it's extremely concerned about plans to replace Luas services with buses on St Patrick's Day.

Transdev announced yesterday that it would hire in Buses this Thursday - in a bid to deal with strike action by workers involved in an ongoing pay dispute.

Talks at the Workplace Relations Commission adjourned for the weekend last night.

The NBRU says it opposes the use of - scab labour - and that it has raised the concerns with the Minister for Transport.