It's an interesting question. From a purely financial side, you should get some projections from a pension consultant to compare a private DC pension. So for example, if you were to put €400 per month into buying back service, what would this get you in a private pension scheme? Obviously, there will be lots of assumptions about growth etc in this calculation, but it will give you something to compare against.
Possibly the bigger question is the question of the risk of a public defined benefit pension scheme. Despite what you read in the Irish Indo, your public pension may be even more risky than a private pension. If the Govt defaults at some stage, you may not get all that you expected. If the Govt changes the policy about how it treats pensioners, you may not get the index linking that current pensioners get. Tread carefully.