If a bricks and mortar shop give it you at the advertised price, even if thats free, its totally at the behest of the shop, be it policy or otherwise. last year I stocked up in superquinn for a barbeque and I had a try of bud in trolley. For whatever reason, the barcode on the tray wouldnt scan and neither would the can, I thought they were joking when the manager came over and said, with a genuine smile on his face, well you get that free then. woohoo. I sent another family member round VERY shortly afterwards (minutes/seconds even) but it had been rectified by then. booo.
I think Tescos do same as they were caught out a couple of years ago when consumers were complaining about being over charged on items, on several sample baskets, there were goods overpriced. when tescos said this was a mathemetical error, they had no answer when they were asked if it was mathematics, how come they were all overpriced and not balanced half over and half under. Methinks they work a lot harder these days on correct pricing.
You will not get same in dunnes.
To the OP, you could mail and ask can something be done as a customer service gesture. Thats about all you can do.