Miners at Tara Mines are balloting for strike action following the introduction of more contract miners at the Finnish-owned plant.
On Wednesday miners held an unofficial sit-in underground to object to the company's action. It was called off on Thursday morning at the request of SIPTU.
The union's regional secretary, Mr Mike Jennings, said yesterday the situation was "very, very serious and one has to ask oneself what the company is playing at".
He said the company was in clear breach of the agreements on the use of contract workers, which averted the closure of the mine last February.
Under the deal, the company had agreed to phase out the use of contract miners by the end of September. However, it has continued to use them since.
On October 9th the company notified SIPTU in writing that 24 Norwegian miners were being deployed. At a meeting on October 10th, SIPTU sought a week's deferral but the union says the miners were deployed the same afternoon.
SIPTU miners responded with a sit-in. Mr Jennings said it was only with difficulty that the men were persuaded to resume normal working while a ballot was conducted for official strike action.
This will conclude on October 20th and is expected to overwhelmingly endorse industrial action.