Staff were yesterday called to a meeting with management who informed them of a "significant development within the company" with new arrangements meaning the loss of the jobs in its warehouse and transport operations.
Sources said the jobs are to be outsourced to the DHL company.
"This announcement has come as a major shock to people in Johnson Brothers and will be a big blow to the workers and the local economy," SIPTU organiser Graham Macken said.
The company announcement will see the workforce reduced to 200.
While the company's 200 other staff not affected by the announcement, they fear what may be coming down the line in the future.
"People are wondering how long they have. We are next," said one worker.
"The atmosphere after the meeting was devastating today. It was dreadful news," he said.
One worker who could lose his job is David Delaney (59) from Francis Street.
"I've worked there 25 years, and there's not a hope in hell of me getting another job. I've a ten year old girl at home," he told the Herald.
"There was a lot of people crying today when they heard the news. It really wasn't expected," David added.
"Some people have been working there 40 years. We'll see what the company come back with to the union, but we're not that optimistic," he explained.
SIPTU said it will meet its members on Monday, and Johnson Brothers' management on Tuesday, to discuss the proposed redundancies.
"SIPTU will seek to explore all options for staff.
"These include redeployment within the company, possible employment by DHL, and, where necessary, acceptable redundancy terms for staff," said Mr Macken.
"We would not rule out trying to keep some of these jobs in-house. Our priority is to keep the work there," he said.
"Management has accepted that it has proposed a collective redundancy situation. It has also agreed to respect its statutory obligation to enter into a 30-day negotiation period during which no notice of redundancy can be served on employees," he added.