Electronics firm Magna Donnelly is to close its plant in Naas, Co Kildare with the loss of 280 jobs.
The company claimed "declining sales, fierce competition and high manufacturing costs" meant that it was no longer viable to keep the facility open.
It said it planned to close the plant by the end of 2007 and would now enter into a 30-day consultation process with its employees.
The first redundancies are expected in March of next year.
The company - which is a subsidiary of US car parts supplier Magna International - has been in Naas since 1968.
Magna plans to transfer all its operations to sister plants in the US and Europe.
In a statement it said: "The decision is a difficult one, particularly considering the impact it will have on employees and the community."
"Employees and their representatives have been kept informed regarding the situation and the possible closure over the last few months," it added.
Minister for Enterprise, Trade & Employment Micheál Martin this evening expressed deep disappointment. He said he had relevant State agencies help those affected to find alternative employment as soon as possible.
A spokesperson for Siptu said the decision would be a blow to workers employed at the plant.
"Our members at Magna Donnelly are deeply upset at this news," branch organiser Joan Wisdom said.
"Many of the employees have given long service to Magna Donnelly, helping to build the brand-name and the profile of the company over the years. Many younger employees have family responsibilities and mortgages to cope with.
"Siptu will now be seeking an early meeting with management to discuss the implications of their decision for the workers and to negotiate the best severance deal possible," she concluded.
Labour Party TD for Kildare North, Emmet Stagg said the announcement was "sadly inevitable."
"This is a dark day for Naas. Magna Donnelly has been a major employer in the area for the last 40 years. Yet the writing appeared to be on the wall for the plant once the company sought 245 voluntary redundancies 18 months ago."