The maximum group discount allowable by law is 10%, and Aviva and VHI offer the maximum 10% discount to the vast majority of people, e.g. through credit unions, trade unions and representative bodies, on-line joiners, most employers, and so on. If you're already in a group scheme and already getting a 10% discount, you cannot get a further 10% discount.
Insurers can decide what groups they offer the discount to, and how much of a discount that group will get, and on what plans they will offer the discount. But beyond that they can't discriminate. If nothing else, the aim of community rating is that all adults on the same plan may the same rate, with allowances for group discounts, student rates, etc.