Cadbury workers in Coolock knew that the company was reviewing its production operations but had no idea of the scale of job losses.
A total of 220 people will be made redundant following a major restructuring in Cadbury's Irish operations across its facilities in Dublin and Kerry.
Staff were briefed by management from Mondelez Ireland, which owns chocolate-maker Cadbury and chewing gum brand Trident.
The company said it plans to close its chewing gum production plant in Tallaght, which will result in the loss of 45 jobs.
An additional 17 contracted roles will also be affected.
It is also expected there will be 160 redundancies in its chocolate manufacturing plants.
These will be from the Coolock factory and in the production unit in the village of Rathmore, Co Kerry.
Mondelez Ireland said its Dublin facility will now concentrate on four of its key Cadbury brands.
This will involve an investment of €11.7m to upgrade existing technologies and production processes.
Richie Browne, regional officer with the Unite trade union, said the number of job losses comes as a blow.
"We knew that the company were reviewing their production operation, they told us that before Christmas.
"We certainly weren't expecting anything in the scale, extent or magnitude of what was announced," he said.
"We're expecting to meet the company early next week and we will be seeking a lot more information and detail.
"We don't know exactly when the redundancies are due to take place and we don't know if they will be voluntary or compulsory," he added.
"We will be endeavouring and trying to mitigate against the losses and to reduce as far as possible the number of job losses.
"The job of government is to support those workers that do lose their jobs by way of upskilling and reskilling and trying to find alternative work," he added.
Mr Browne said he had no direct contact with Cadbury management since the announcement of the job losses yesterday.
The Cadbury factory in Coolock is to lose a substantial number of jobs.
The planned redundancies are seen as a major blow to the economic life of Dublin's northside. The plant has been a beacon of manufacturing employment in the area for more than 50 years.
The workforce will be reduced as part of an overall efficiency programme being implemented by the company.
It is planned to set up a series of consultation meetings with staff and their union representatives. A SIPTU spokesman described the announcement of the job losses as "a complete shock".
The union has also alleged the company intends to move some of its chocolate production facility from Dublin to Poland in the near future.
The Rathmore plant, which has long been a staple of manufacturing employment, was opened in 1948, and the Coolock unit has been operating since 1964.
Changing consumer patterns means that Cadbury's iconic pink Snack, and its equally well-known Time Out bars, may no longer be produced.
However, other well-known brands, such as Dairy Milk, Flake, Twirl and Boost will still be manufactured both for domestic and export markets.
SIPTU organiser Michelle Quinn said more time is needed to consider all the issues involved if the Tallaght closure is inevitable.
"The threatened closure of this plant, along with the Mondelez Ireland announcement that it is seeking to cut 160 jobs at other plants, amounts to a dark day for manufacturing in Ireland," he said.
"These job losses will severely impact local communities," he added