We are serious milk drinkers in our house, and I have noticed that the milk from stores, other than Dunnes, besides sometimes tasting different, it also goes off much sooner, especially Aldi, Lidl and Tesco.
I drink 1-2 litres of milk a day. Sometimes I will carry a litre for a whole day in my bag, end up not needing it, and put it back in the fridge and it's still perfect a couple of days later. Every so often, I have to travel for work at short notice and arrive back to a few litres that are up to a week out of date. Again, they are perfect - taste normal, normal texture.
I will often buy a litre from wherever I pass when out and about so every month would have a mix of Aldi/Lidl/Dunnes/SuperValu/Tesco/Centra own brands and sometimes Avonmore. There are taste differences (Jersey cow milk is #1, indisputable) but my experience in terms of spoilage is similar.
I hear a lot of people talking about gone off milk and it makes me wonder why some have so much trouble. I do notice people will leave milk out on the counter for ages, so it gets warm and then is put back in the fridge, cycle repeating frequently. I also see people drinking directly from the container and putting it back in the fridge, which introduces bacteria. Or maybe it's cold chain issues local to stores?