More than 100 jobs are to be lost at a leading healthcare employer.
US company Baxter, which makes kidney dialysis products, is one of the biggest employers in Mayo with plants in Castlebar and Swinford.
It is understood management want to reduce the number of shift workers at the weekend.
Staff were notified of plans for 110 voluntary redundancies at briefings over the last few days, with all voluntary redundancies being sought in Castlebar.
Baxter has been in Mayo for more than 40 years and at one stage had three bases in the county and is one of the biggest employers in Castlebar, Taoiseach Enda Kenny's home.
Frank Jones, organiser with Siptu's pharmaceutical and medical devices division, is planning to hold talks with management tomorrow.
"It hasn't come completely out of the blue. There have been a few leaks of late but we weren't prepared for the entirety of the potential job losses or the short term of the plan to secure the redundancies," he said.
"There's people there today feeling very, very vulnerable, people who are coming off the night shift and panicking."
Management are understood to be aiming to cut the number of staff on the weekend shift from 180 to 70 and t he redundancy plan is expected to be complete by next March.
Baxter has about 1,000 employees across its sites in Castlebar and Swinford.
Siptu will hold talks with management tomorrow morning as the redundancy process gets under way.
"The purpose of this meeting is to explore the potential to reduce the number of proposed job losses and to try and ensure that no compulsory redundancies are implemented," Mr Jones said.
Baxter's cuts come less than a week after global pharmaceutical company MSD announced plans to shut its plant in Swords, Co Dublin, with the loss of 570 jobs.
Dara Calleary, local Fianna Fail TD, said the decisions raise concerns about the future of the healthcare sector in Ireland.
"This news is particularly upsetting in the lead-up to Christmas," he said.
"The staff must be given every support possible as decisions are made about voluntary redundancies. Baxter has been a major employer in Co Mayo for decades and many of the staff will have worked there for years. Everything must be done to ensure that they are offered redundancy terms that reflect their years of service and loyalty to the company."