Workers at Quinn Insurance believe that up to 1,000 jobs are now at risk at the troubled company.
The Republic's Financial Regulator has allowed the firm, which is in administration, to only partially re-enter the UK market.
As a result the company's administrators said on Thursday night that some job losses are "inevitable".
Quinn employees said that it was a devastating blow and signified the start of the redundancy process.
Brian Storey, a member of the employees' committee in Dublin, said the effect of restricting the UK business would be "catastrophic for staff".
The company, which employs 600 people in Fermanagh, was placed into administration last month and was prevented from doing new business in the UK.
Most of the UK business is processed in Quinn's Fermanagh operation and the ban had put hundreds of jobs at risk there.
The regulator acted due to concerns about the financial position of the company, particularly in relation to its solvency.
The root of the company's problem was a massively unsuccessful stock market gamble by the company's owner Sean Quinn, which lost him hundreds of millions of pounds.
On Wednesday, the regulator eased restrictions placed on Quinn Insurance that prevented it from issuing new insurance cover in the UK.
The regulator said it would allow the firm to write new insurance cover for provisional licence holders in the United Kingdom, but that is just 10% of its UK business.
Joint Administrator Michael McAteer said they welcomed the decision to allow a limited return to the UK market but said they had not yet been able to secure the reopening of other areas of the business.
"As a consequence the UK businesses will be reduced significantly compared to the levels prior to the Administration.
"We are currently assessing the impact on the business and particularly staffing levels in the short, medium and longer term. Regrettably we cannot continue as we are."