If he has been on systematic short time (which it appears he has) for more than 4 weeks, then he is entitled to apply for voluntary redundancy. He does this by submitting a form RP9.
If you can offer him 13 weeks work on full hours, then you can put a counterclaim against his claim for redundancy. If you cannot offer him the required amount of full time hours, then you have an obligation to pay the redundancy. Any redundancy that is paid should be based on his full-time hours, not the reduced hours.
If you consider that you cannot afford to pay the redundancy, the employee can make a claim for the money from the social insurance fund. To claim an inability to pay you must submit a report from your accountant saying that this is the case, and that you accent liability for 40% of the payment. Whether you can afford the payment will be assessed by the Department. If they agree with you, then they will make the payment, if they don't agree, then the liability remains with you. This takes about 6-9 months for the Department to process these applications.
For more information see the following guide to Redundancy [broken link removed]