The n/80ths rule is based on a factor of 9 to 1 for converting pension to an equivalent lump sum. This factor no longer applies and the pension equivalent of a lump sum can be calculated by reference to immediate annuity rates at the member‘s date of retirement.
It’s probably simpler to look in the first place at the maximum pension that can be provided prior to any reduction in respect of a lump sum taken on retirement. For people retiring at normal retirement age, the maximum permitted pension is 2/3rds of final remuneration for a member who has completed at least ten years of service. It is reduced pro rata for members with less than ten years service and is inclusive of any retained pension benefits from previous employment or periods of self employment. On early retirement, the above maximum benefit is reduced in line with the ratio of completed service to potential service to normal retirement age.
Normal retirement age can be anywhere between 60 and 70, with earlier ages permitted for certain occupations (e.g. certain sportspeople). If we take your example of the member who joined at age 30 and retires at age 55 and assume that their normal retirement age is 60 and they have no retained pension benefits, then the maximum pension they can have at age 55 is 2/3rds x €60,000 x 25/30 = €33,333 per annum. This pension would then be reduced to take account of the pension equivalent of any retirement lump sum. For the member in question, the maximum permitted early retirement lump sum would be 120/80 x €60,000 x 25/30 = €75,000.
Hope this helps.