As in all good shows or even in real life, new police can come in to clean up the previous administrators clutter and solve cold cases or even reopen bounties that were previously dismissed. It's the prerogative and rp of the garda in question to do this now as the offices of garda before her have been a circus to someone driven by law and order, especially order.
That dynamic is not assured. In real life, killers can be caught even decades later. In game, just because something became old does not mean it is forgotten, if someone is gun-ho