I would like to ask if the tap water in hostels of Ireland and England is drinkable. I never saw someone drink from there, and even in the pubs you have to order for a bottle.
When I moved to London for a few years, almost all my friends had filters. The trace calcium / limescale in the water will result in some people telling you it's "hard" water, or "soft water" depending on where in the UK you are.
Indeed, there's debate over where it tastes better or worse:
Many friends had these filters to try and remove the limescale, and you had to regularly clean your kettle to remove the build up of this stuff.
However, is it drinkable? Sure. No problems at all. Most who buy the bottled water did so purely for convenience, or taste.
Short answer - you'll need to ask the property to be sure.
All the mains tap water will be fine to drink everywhere in the UK and Ireland.
However... Some hostels may have a tank which feeds some of their taps, so you may find that the taps in the kitchen are mains-fed and fine, but the ones in bathrooms (for an example) could be fed via the tank, and may not be as fresh. The hostel will be able to tell you which ones are fine. Additionally, there are some hostels that aren't on the mains water (eg remote ones in Scotland), so there may not be any drinking water there, other than what you hike in yourself.
In pubs - it's always fine to ask for a glass of tap water, and they'll give you one (it's in their license). However, bottled water may taste nicer, so people not drinking eg beer may well often prefer bottled.
Premises that serve alcohol are obliged by law to provide free tap water to guests so I wouldn't be shy to ask for it. Nowadays they don't even look surprised when you order mains water!
BTW here is a list of some FAQs regarding British tap water (including your question). It's not hyperlinked unfortunately but I used the ctlr+F function to look up this answer
The water from taps in Great Britain does vary in hardness depending on where you are as the map above shows. Anywhere where food is involved the water will be mains supplied and quite safe to drink. Mains water is carefully and continuously monitored by the Water Authorities. This does not apply to water in washrooms/toilets etc. Often this water is supplied from a storage tank in the attic of the property and is not potable (drinking). This practice is rarer in modern properties but is quite common in older buildings, small hotels, lodgings etc.