A total of 189 full-time and 137 seasonal jobs will be lost.
The company also announced yesterday it will consolidate all sugar manufacturing at its Mallow plant, to be upgraded to process Ireland's 199,260 tonnes a year sugar quota.
Irish Sugar said closing the Carlow plant and upgrading the Mallow facility will involve a net investment of between E20 million and E25m.
The firm's workforce will be reduced from 614 to 288. A total of 35 of the full-time jobs and 16 of the part-time jobs affected will go through a voluntary redundancy programme already agreed for the Mallow factory.
Irish Sugar said the successful implementation of the rationalisation plan will help secure the ongoing viability of beet processing for its remaining staff and some 3,700 beet growers.
A total of 63 staff jobs will be retained in Carlow in sales, marketing, distribution, sugar packaging and administration.
The company said the move was designed to ensure the survival of the business in the face of threatened reforms of the existing EU sugar regime and the increasing competitive nature of Irish Sugar's market.
It said the retention of two plants was no longer viable. It would cost 28m less to upgrade the Mallow facility than it would take to establish the Carlow facility as a single operation. The rationalisation will enable it to reduce its annual cost base by some 6 to 7m (net) over the next number of years.
Chief executive Dr Seán Brady said Mallow emerged from a review as the most suitable option across a range of criteria including a lower "go forward" investment requirement and the presence of a railhead, which will allow up to two-thirds of Carlow beet supply to be transported to the facility by rail.
Greencore chief executive David Dilger said the decision had been particularly difficult and painful but, nonetheless, unavoidable.
There was relief in Mallow that the plant was not facing the axe.
However, Carlow Kilkenny TDs Phil Hogan (FG), MJ Nolan (FF) and Séamus Pattison (Lab) described the decision to close the Carlow plant, as unjustified.
SIPTU regional secretary Mike Jennings said the closure was a disgrace.