Unless the law's been changed since, the general rounding rule in Ireland since the euro changeover is that .49 or less rounds down, .50 or greater rounds up.
Therefore anything with a cash price ending in 5 cent can be paid for as is, 6 cent rounds up to the next 0 and 4 cent rounds down. Theoretically. Since this scheme is voluntary, the retailer may choose to accept 5 cent for the 6 cent item.
Also, if there's more than one item rounding has to be done once, on the total price and not on each item.
IANAL, but that's what we were told at the time of the changeover. I ran the conversion for one small part of the postal service. We, and the Revenue, and I assume all other government departments were told - and I mean ordered, not "advised" - that a) the conversion better go perfectly, b) in strict accordance with the rules and c) no member of the public was to lose out. If there was any doubt, round in favour of the customer.
You wouldn't get that attitude now. These days it's up to you to get away with whatever you can.