It's not just that though. The funds have to be lodged with solicitor A from the purchaser, once that money is cleared, then he transfers to Solicitor B, if there's a weekend there it might cause problems. Or if the person doing the transfers is off sick or on holiday, that could delay seeing the money was actually there. Then the money has to be transfered out to the vendor.
But if you ask the question how long does it take from one Irish bank to another, a couple of days. But if it's the same bank than it would be the same day.