SIPTU has said it is has been told at least 27 workers are losing their jobs at the Liebherr manufacturing plant in Killarney.
The firm employs more than 800 people making cranes.
The union says it was told 27 workers will be let go this month and there could be further jobs lost in July and August.
It has described the news as a severe blow to the workforce.
SIPTU Organiser Joe Kelly said: "Our members are shocked and disappointed at the confirmation by the management of Liebherr of the extent of potential job losses at the plant over the coming weeks and months.
"SIPTU representatives were informed yesterday of the scale of potential job losses with 27 workers to be let go at the end of June, up to 60 in July and possibly 128 in August.
"This is a severe blow to the loyal Liebherr workforce, their families and the community in Killarney and surrounding areas.
"The company has provided steady employment for generations of workers in Kerry."