Can they do that???! Or is employment law different in Ireland?
Pretty sure they can't (again.. awaits Cesare

)
Oops, soz
I don't know about employment law in the ROI but I doubt it'll be hugely different to the UK because of the overarching EU aspect.
In the UK, a redundancy situation happens when there is a ceasing or diminishing of work.
All affected employees have to be meaningfully consulted about how to avoid or minimise the effects of redundancy & selection criteria. If the numbers go above 20 within a 90 day period, then collective consultation kicks in - union/s and elected employee representatives.
There are no rules for selection criteria, that's a matter for consultation. Many employers have ditched the old LIFO criteria (Last In First Out) in favour of retaining the best skills. But there are some employers that use LIFO up to one year's service, because employment protection against unfair dismissal is triggered at one year.
Sometimes volunteers are encouraged, but employers generally reserve the right to refuse a voluntary application if the employee is key, or the severance cost would be too high.
Selection criteria - often a matrix is used. Criteria might include skills, application/attitude, disciplinary record, attendance record (discounting disability and/or family leave related), etc with LIFO being the decider where the scores are the same.
Redeployment and retraining have to be considered.
The six month thing is a myth. There's no legislation that says that employers have to wait for 6 months before re-employing. I think that myth has arisen through practice ... waiting until time limits for making claims for unfair dismissal/unfair selection for redundancy claims have passed.
Good luck to all who are facing this.