The pension benefits from the Single Scheme are less attractive than for previous public service pension schemes. The benefits are based on career average earnings rather than final salary. Also, you can no longer purchase "notional years" as such, although you can purchase additional retirement benefits. You can also, transfer in from other non-public service pension schemes. I have scanned through some of the documentation and the process for either/both of these does seem fiendisly complex (or, to put it another way, I don't understand it !).
I don't know which of the options referred to by LD above you are considering. Just to note, though, that a further option might be to go with an AVC or PRSA-AVC along with your public service pension. This might be more flexible at retirement as you can use the fund to maximise your tax-free lump sum and then transfer any residue into an ARF to be drawn down flexibly.
By the way, have you read the Circular on the Transfer and Purchase Schemes?
https://singlepensionscheme.gov.ie/...-the-Single-Public-Service-Pension-Scheme.pdf