Up to 200 SIPTU members picketed the Custom House in Dublin city centre this morning as part of a dispute involving cleaners.
The picket was arranged to show solidarity with 10 cleaners who are losing their jobs under a new cleaning contract for the building, which houses the Department of the Environment.
SIPTU says the company that secured a new cleaning contract for the facility is failing to honour commitments to staff.
The union says security staff locked the building from the inside during the protest, preventing all other workers from gaining access for around two hours.