Unite's 108 members at the site are in the midst of a month-long work to rule and overtime ban in a bid to get two dismissed colleagues, a deputy senior steward and a senior steward, reinstated.

According to Unite, the two dismissed employees were sacked after they "stood up to the company over 'race to the bottom' contracts and alleged incorrect bonus payments" that could cost Smurfit Kappa more than £250,000 pending the outcome of a forthcoming court case.

In addition to its ongoing overtime ban and work to rule, the union has announced plans for a four-day strike, to run from 17-21 April, in an escalation of an earlier 48 hour-walkout (on 1-2 April), which failed to get the two staff reinstated.

Unite said that it was ready to hold talks with the company at any time, under the auspices of ACAS, but that thus far the company's management had "maintained its hard line".

Unite regional officer Mike Orpin said: "Our members are resilient and steadfast in getting our two dismissed reps back to work and have decided to escalate the action to a four-day stoppage starting next Friday.

"This strike will run in tandem with the month-long work to rule and overtime ban which finishes on 24 April. It is anticipated that all our members will be taking part in this escalation.

"We are still hopeful that the company will agree to meet with us and enter into positive discussions – but, at present, it is maintaining its hard line. We call on Smurfit Kappa to reinstate the two stewards immediately.

"Both stewards were instrumental in bringing a claim against the company for the incorrect payment of a bonus, going back a number of years, and could cost the company over £250,000, depending on the outcome of a forthcoming court case.

"They were also instrumental in resisting second-generation contracts being introduced into the workplace in 2014, which would have meant new starters getting paid less than existing workers for the same job."

Smurfit Kappa was unavailable for comment at the time of writing.