Pickets are to be placed at Waterford Regional Hospital today in a dispute over the alleged displacement of security staff.
Siptu claims a company which is to take over the security contract from today has refused to employ the existing staff.
The union said yesterday the company, Securway, had informed it new staff were to be brought in on lower rates of pay than those that had applied to date.
Securway was awarded the contract after a tendering process, and replaces Sentry Security, which had provided the service until the weekend.
Employees of Sentry Security, who are members of Siptu, plan to picket the hospital from 8am.
A spokeswoman for the Health Service Executive-south said the hospital had been informed of a plan by Siptu to place "secondary pickets". A full security service would be provided to the hospital and no disruption to any of its services were expected, she said.
Siptu south-east regional secretary Mike Jennings said Sentry Security had had the contract for the past seven years.
He said the union had contacted Securway to try to ensure the existing staff were taken on, as provided for by transfer of undertakings legislation.
Securway had maintained that the legislation did not apply in this case, but this was disputed by Siptu, he added. Securway's management could not be contacted yesterday for comment.