In the Programme for Competitiveness and Work (PCW) Agreement it was agreed that some service given by public servants, previously not recognised for pension purposes could now retrospectively be recognised as pensionable.
This situation would apply to situations such as if a member's contributions were refunded if they left service or perhaps they worked in a part-time capacity that was not at the time recognised as pensionable.
Such years can be bought back. Typically such service must be paid for at retirement if it has not already been bought back. In almost all instances it is a recommended that this service be bought. This is because such years are heavily subsidised by the employer. In effect a member's return on investment or "break even point" is achieved relatively early into retirement.
PSRA can give you a estimate of the costs and benefits involved however if you feel you have previous service that may be allowable for purchase under the PCW agreement you should contact you employer's pension department.