Still waiting on HR shared Services to action my request. Been 2 months now so will probably just raise a new ticket.
I'm fairly sure you pay the up front cost in full to payroll and then recoup the Income Tax from Revenue.
For any unpaid leave, such a Shorter Working Year, Career Break and Parent's Leave, you can buy back the service. You must do so no longer than 6 months after returning from that leave. It's not cheap so a few weeks is doable but anything more is v pricey