Any steer on this would be much appreciated.
I don't know the answer.
It was always my understanding that the purchase of notional years did not count towards the Supplementary. Class As pay towards two different pensions, which are coordinated - the Occupational Pension and the State Pension (via PRSI). If someone retires after normal retirement age with 30 years service they get 30 years worth of Occupational Pension. If they do not also qualify for 30 years worth of the State Pension ("through no fault of their own") and they are not working elsewhere they can apply for a Supplementary Pension to equal value, ie, their Supplementary will be 30/40 of the State Pension. If this same person had bought 10 notional years they would add to their Occupational Pension proportionally. But they have not paid 10 additional years of PRSI (or equivalent) and therefore the Supplementary pension remains at 30/40 of the State Pension.
As I say, that was always my understanding (and one reason I preferred AVCs for myself). However, since someone recently pointed to the Dept of Ed Modeller indicating something different I just don't know any more. And, on the other hand, some other poster has referred to getting a similar steer to your self from their employment office.