Under the current “”totals and average” system you need a minimum of 520 A Stamps (10 years) to get the minimum pension (40% of State Pension). Not sure however that you would get the full State Pension if you first started contributing to A Stamps at 55 having previously been a D or D Stamp. The current system is based on when you first entered the PRSI system (whether A or B/D).
It is true to say that if you emigrated into Ireland at say age 55 and worked in the private sector (A Stamp) for say 10 or 11 years then you would qualify for a full pension , because your “average” contribution per annum would exceed 48. This is an anomaly in the current system.
HOWEVER, all this is due to change in the next 2 to 3 years when the calculation basis is expected to move to a 1/40th of State Pension for each year of contribution. So I expect your query will be academic in time..