The administrators of Quinn Insurance Ltd are to make 800 staff redundant, 200 of those in Northern Ireland.
It is part of their plans to downsize the firm, and affects about one third of the staff employed by the company on both sides of the border.
About 200 will go from the company's operations in Enniskillen and Derrylin leaving a combined workforce of 450.
The remaining jobs will go at the offices in Cavan, Navan, Dublin and Manchester.
It is believed the administrators, now running the company, hope the redundancies will be voluntary and take place gradually.
Quinn workers' spokeswoman Mona Bermingham said staff were extremely fearful for their futures.
"This is the first we've heard of it - we had been given no indication of numbers or locations," she said.
"There's a lot of worry, disillusionment, dejection - it's going to have a massive impact and it's not something we'll be accepting easily."
In April, Quinn Insurance was placed into administration after the Irish financial regulator expressed concerns about the company's solvency.
The company was also ordered not to write any new UK business. Those restrictions were later partially eased, allowing the company to offer cover to provisional drivers.
On Thursday, a committee of the Irish parliament backed calls by employees to allow the company more access to its markets in the UK.
Five members of the workers' action group told the Enterprise, Trade and Employment committee that the company is losing 2,000 customers a day in the UK.
In Northern Ireland, the Assembly committee on Enterprise, Trade and Investment met with the administrator.
The committee says it also pressed for Quinn to be able to access UK markets.
However it said it had not had any discussions with the administrator about staff redundancies.
The firm's problem stem from massive losses made by its owner Sean Quinn in a stockmarket gamble on the now-failed Anglo Irish Bank.
Although the job losses will mostly be in the Republic, the restructuring relates primarily to Quinn Insurance Ltd's businesses in the UK.
Much of the UK business is handled by staff at Quinn's offices in Dublin, Cavan and Navan.
The details of the package to be offered to employees have not been revealed but it is understood workers will be offered more than the statutory redundancy.
In an email to staff the administrators said they were "extremely disappointed" that details of the job cuts had been leaked to the media ahead of an official announcement on Friday.