Impact has instructed its members in the Western Health Board not to provide cover for 169 jobs - mainly clerical and administrative, which are to go in the WHB's cost-cutting campaign.
Meanwhile, the Irish Nurses' Organisation is to launch a nationwide campaign for a 35-hour working week.
Theatre nurses at Mullingar General Hospital have suspended a work to rule following acceptance of a Labour Relations Commission recommendation on staffing levels.
IMPACT is to meet the WHB tomorrow over the job cuts announced at this week's board meeting.
The union's national secretary, Mr Kevin Callinan, said the WHB had already written to some temporary staff to say that their contracts would not be renewed "despite the union's request that they hold off until full consultation had been exhausted".
Mr Callinan accused the board of "seeking a de facto reduction in jobs and services under the cover of consultation".
IMPACT's instruction to members not to cover the posts being cut would expose the impact of health cuts on services, he said. "The effect of these cuts will go much wider than the individuals involved. They will affect everyone who works for the Western Health Board and everyone who uses its services.
"It means doctors, nurses and other health professionals will end up spending more time on administration and clerical work, and less time treating patients."
Asked to comment, the WHB said all issues would be discussed at its meeting with IMPACT tomorrow.
The Irish Nurses' Organisation's campaign for a 35-hour working week is part of its response to the report of the Benchmarking Body. The INO had sought a pay increase of 30 per cent but the increase recommended for staff nurses is 8 per cent.
The INO holds a special delegate conference at the end of this month to decide on its response to the benchmarking recommendation. In an editorial in The World of Irish Nursing, the INO general secretary, Mr Liam Doran, has warned members not to allow themselves to become isolated in fighting the recommendation "while the rest of the public sector unions look on, ready to enjoy whatever hard-earned gains we make without any pain whatsoever".
In the first instance the issues will be brought to the Labour Relations Commission. The INO has so far not said what it will do if it fails to get the response it wants at that forum.