You wont be able to cash in your pension until you are aged 50 at least or if you have to retire on ill health grounds before then.
If you go the PRB route you will be able to cash it in when your 50 regardless of what age you actually retire at. You will not be able to add extra funds to this when you return to work so you will need to take out another pension contract.
If you go the PRSA route you will not be able to cash in until age 60, again you may cash earlier if you retire on ill heatlh grounds. If you decide to retire at age 50+ you can then cash in your pension