Well, as with any utility, the buyer will only be liable for any bills from the date they purchase the property - historically no checks are done for outstanding bills for gas/electricity, so you won't even know about any outstanding bills until you take ownership (I'm speaking from experience on that one as it happened to me when I bought my current place, I got landed with a gas/electricity bill in excess of €850 that the previous residents left unpaid ... once I provided proof I had just taken ownership, it was sorted). So I don't see why an Irish Water utility bill should be treated any differently; indeed, if I've not registered, then there isn't even any question of a bill ...