The IMPACT trade union has begun a campaign of industrial action this morning as part of a dispute over the Government's planned reform of the health service.
Members of the union, which represents 25,000 health workers, have vowed not to co-operate with the new Health Service Executive set up to implement the reforms.
The workers want assurances about job security, working conditions and continuity of service before they will sign up to the changes.
Spokesman Bernard Harbour said: "IMPACT is not opposed to the health reforms and we've been supporting the process, but the reason we're taking this action from this morning is because we haven't had the safeguards that staff are looking for about their job security, about their work location and about their working conditions.
"We've been in discussions for a number of months to try to achieve acceptable safeguards, but we haven't had that yet and that's why the action's going ahead this morning."