I hope the state does recoup this money as those of us who are employed at low levels in the infamous public sector have many stories to tell over the years of errors on payment side which regardless of whether or not one realised it , had to be paid back.
In fact a friend was on wrong PRSI rates for years and didn't know it until he retired ;on discovery it had to be paid back. Same thing happened to another friend who was paid overtime. In fact I make a point of checking up if there is an unusual increase in my salary. In fairness, when these mistakes occur, there is always negotiation re repayment but it is never just written off. Hopefully that's not just for 'small' people.
That's why I can't understand why in some of the recent scandals re expenses that are exhorbitant, the paying authority is not demanding monies back. It wouldn't happen in the dept I work in and as regards travel and sub ,it's so much hassle to get it , that I haven't bothered claiming and have taken time in lieu instead.Granted I don't travel much but to be honest my mental health is more valuable to me than the hassle that ensues with claims where I am. Pity that level of bookeeping didn't go on everywhere.