Workers at Element Six are voting on industrial action after talks on a survival plan for the firm between unions and management ended with resolution last night.
The troubled diamond manufacturer is proposing to keep 80 research and development jobs at Shannon, with the possibility of a further 163 if a survival plan can be agreed.
The results of the ballot should be known tomorrow afternoon.
Up to 370 jobs are at risk at the factory if the proposals fail.
The current stumbling block is the issue of redundancy payments, which the unions want improved.
"No progress was made because local management said they were not in a position to agree any improvement on the offer of two-and-a-half weeks per year of service, plus statutory entitlements," Siptu Branch organiser Mary O'Donnell said today.
"This is totally unacceptable given that six weeks pay per year of service has been the accepted formula going back to the 1980s and was being paid to people who opted to leave as recently as last March."
However it has emerged that the management team at the company has written to the group's chief executive Cyrus Jilla in London, urging him to engage with workers on the redundancy deal on offer by the company.
The letter to Mr Jilla said the plan to trim the Shannon operation was affecting trust between the company and its employees, and was also damaging customer confidence in the company as "a reliable supplier with a sustainable future".
"We have a plan for a long term, sustainable operation in Shannon," the letter said.
"In order to minimise further disruptions to the business, we believe that it is imperative that the company engages, urgently, in meaningful discussions with the Shannon workforce that must include the severance package to achieve a resolution as quickly as possible."
Regional secretary of the Technical Engineering and Electrical Union Pat Keane said there could not be "meaningful discussions" on the survival plan for the plant until it was known on what basis most of the workers would be let go.
"The company's decision to cap maximum redundancy payments at the equivalent of one year's pay is a particularly penal move, given that all of my members have over 25 years' service," he said.
"Under the previous package the cap would have been the equivalent of 2.5 years pay and my members would have qualified for the higher payment."
Element Six employed 1,000 workers at its peak, but numbers have fallen to 450.