Element Six has blamed 'the high cost of operating in Ireland' for its decision to end manufacturing at its plant in Shannon, Co Clare, with the loss of 370 jobs.
The announcement was made this morning at a meeting with workers.
All manufacturing and distribution is to be discontinued with only 80 workers being retained in the research and development area.
SIPTU has condemned the redundancy offer to the 370 as unacceptable.
It said workers were angry at the offer to give them one week of pay per year of service on top of statutory entitlements.
The union said on previous occasions the company had given six weeks of pay, inclusive of statutory entitlements of two weeks.
The company said the high-cost environment in Ireland could no longer be sustained.
Element Six said the Shannon operation, which operated a central marketing and technical support unit, was the most expensive manufacturing unit in the entire global operation.
A statement from the company read: 'Despite a series of cost reduction programmes in the past few years, continued production at the Shannon site is no longer sustainable or viable, and the primary business it serves is loss-making.
'The current economic downturn has simply served to exacerbate these challenges.
'This is a highly regrettable decision for Element Six and a number of alternatives were considered at length.
The company announced 150 voluntary redundancies last December; a scheme that is believed may have been oversubscribed.
The company is one of the world's biggest manufacturers of supermaterials and industrial diamonds.
It also has plants in South Africa, Sweden, Germany and Israel.
It is one of the biggest and longest-established companies in Shannon and was formerly known as DeBeers.
Shannon Development, Chief Executive, Dr Vincent Cunnane, said the closure decision was a severe blow to the workers and a major disappointment for Shannon and the wider region.