Dunnes sold expired chocolate

Probably. I've always found the major supermarkets to be very accommodating in these scenarios.

Did you eat it all?

I reckon you can in that you could have dumped the chocolate because it was vile - and just present the wrapper and receipt clearly stating purchase date.
I’d doubt they will quibble over 3.50

Saying that the chocolate itself shouldn’t have tasted that bad - was only a few weeks out- are you sure it’s not just bad tasting chocolate in general?

Just take it back to customer service with receipt and they will refund you.

There's no such thing as out of date chocolate. There is chocolate that doesn't taste or look as good as it should after a while but it's not dangerous or risky to eat if you've no other health issues.

Best Before Date is to inform you that it might not be at it's best quality if you consume it after the date but the product is still perfectly safe.

Use By Date is to inform you that it is dangerous to consume after the date. And even at that some Use By Dates can be way off because eggs can last a lot longer than their use by dates and are easily checked to see if they are still safe.

Why is selling something past it's Best Before date bad practice? It's not an expiry date so there's no issue with selling something past it's Best Before date, this is what's leading to people dumping perfectly good food because they think that Best Before is an expiry date.

Lidl do as you are suggesting, they put an orange sticker showing a discount off the shelf price on the likes of sliced bread and tubs of coleslaw if the item is getting close to it's expiry/best before date. I don't think Dunnes do anything similar.

I buy the Lidl 'Seed Sensation' brown sliced pan for toast so I don't need it to be bakery fresh and will buy a discounted sliced pan if one is available.

Use by date is legally binding. BB is not. While it is normally accepted that shops will and do, give a refund if its is past BB date, it is not legally binding. It is still perfectly ok to sell and consume.

Shelf-life: Best before and Use by Dates | Food Safety Authority of Ireland (fsai.ie)

I didn't say it was bad practice or illegal.

I said it wasn't best practice.

As a customer I wouldn't expect to pay full price for an item that was 6 weeks past it's BB date.

If it was offered at a discount I could decide whether it suited me to purchase it or not.

I object to having such an item passed off to me because the retailer over ordered or failed to rotate stock properly.

comparing a bar of chocolate to a brand new car 😂😂😂