Not completing answering the question here, but am happy to confirm that in England and Wales you can hold power of attorney once discharged. Interestingly, if the power of attorney is created prior to the bankruptcy then it can continue during the bankruptcy. Only people who are currently bankrupt are restricted from taking up power of attorney for somebody else.
Chances are that the rules are similar for Ireland. From a practical viewpoint, your wife has been discharged, she is not bankrupt, more than likely solvent, don't see any reason why she shouldn't be able to take up power of attorney for your father in law.