Even if they did default on the interest, or indeed on paying out the face value of the bonds themselves, the debt would still be due. In order to ever borrow money again, they would at some point have to honour the existing debt. Whatever about international debt holders, domestic investors in An Post savings bonds and certificates would have to be paid.
Otherwise, Ireland.Inc would be unable to borrow. Too great a risk. They'll think to themselves - if they won't pay their own taxpayers and citizens back, I'm not going to be paid back. So they are not going to lend to them either.
Countries have tried renouncing debt before - I read somewhere (correct me if I'm hazy on the detail) that the Soviet Union renounced the debts of Tzarist Russia, bonds in particular, but found that it had to pay at least some of the debt back in order to get access to the international credit markets.
I consider it unlikely in the extreme that Ireland would default on any of its bond committments, to be perfectly honest. Bad and all as things are, they're just not THAT bad.
But even if it did happen in some doomsday scenario, at some point to effect any kind of recovery, they would have to pay back some if not all of the outstanding debts which they defaulted on. So in that highly improbable scenario, at some point you would see a return.