Staff who transferred to Irish Water from local authorities kept their original pension schemes. New employees were offered a defined contribution scheme. As you describe it, it sounds like your friend would be leaving the Civil Service and taking up a job with Irish Water (rather than their post being transferred to Irish Water). If this is the case I guess they would be joining the defined contribution scheme.But they should check out if this is so in their own circumstances.
If someone leaves the public service he/she can (and must, I think) rejoin their original scheme if and only if he/she rejoins public service employment again within 26 weeks. Otherwise they join the Single Scheme (ie, the post 2013 scheme).