She can take early retirement from a PRSA from 50 onwards if she leaves the PAYE employment where the PRSA contributions came from.
From 60 onwards she can retire the PRSA while continuing to work in the job if she chooses.
Even if she selects 60 as the chosen retirement age now, she can change that at any time to any other age between 60 and 75. Equally if she selects 66 as the retirement now, she can still choose to retire at any age between 60 and 75.
If she chooses a default investment strategy which dials down the risk as she approaches retirement age, the strategy will assume that she's retiring at whatever age she put down on the form. So that's probably the biggest implication of the selected retirement age.
Regardless of what age she retires the PRSA, she can take 25% of the fund as a lump sum (tax-free up to €200,000). With the other 75% she either buys an annuity or reinvests in an ARF and ARF withdrawals are her pension. Either way, annuity income or ARF withdrawals might be taxable depending on what other income you have as a couple. Normal Income Tax rules and rates.