1. If they have no class A earnings in the previous year, they will pay the minimum 500 euro.
2. They can stop payments for any year and restart again any time before age 66.
3. No need to backdate. But they could backdate if it was advantageous to them.
4. The voluntary contributions are based on a calender year. Jan - Dec. So if a person worked for 26 weeks from the start of a year and then made voluntary contribution for the remainder of the year they would just get 26 voluntary contributions for that year.
5. They just cover for reckonable pension purposes.
A person making voluntary contributions is also allowed to remain claiming Jobseekers credits. If they continue signing for credits they will maintain their treatment benefits entitlement. The credits are also useful if the wish to qualify for Benefit Payment 65.
A person with 20 years of paid class A contributions might not be advantaged by making voluntary Prsi payments.
Credits might be of equal benefit to them at no cost.
If the person's pension is calculated using the total contribution method they can have up to 520 reckonable credited contributions. This increases to 1040 if they also have homemakers credits.
If their pension is calculated using the yearly average method, there might be no limit on the number of credits allowable. (I am not certain about this.)
Operational Guidelines: PRSI - PRSI Voluntary Contributions.