I don't know whether England is more expensive than Ireland, though as someone living in England it certainly feels expensive to me.
But I think it's difficult to make a proper comparison unless you're maybe splitting your time between the two places fifty fifty (living in one place for a few years and then the other for a few years wouldn't work so well, as things would be changing in the other country over that time).
At home, over time you gradually get a feel for what's pricey, what's worthwhile, what's not worthwhile, whereabouts it's most worth putting in moneysaving efforts for a saving that's worth getting, and you perhaps almost subconsciously adapt the way you do things, what you choose to do, and even maybe the things you prefer, around that particular price landscape.
So maybe in a country where coffee shops are an expensive upscale kind of venue, but tea shops are low-cost places that ordinary people treat like community hubs, you'll develop a tea habit and rarely go for coffee. If you travel to a country where coffee shops are cheap, boisterous, friendly joints that anyone might go to, and tea shops are sophisticated, classy, and very dear, you'll make your habitual stop for a quick hot drink at a tea shop, and marvel at how expensive this other country is, when in reality, people like you who live in that country frequent the coffee shops instead. Or at home, you instinctively know that the bus is likely to be cheaper than the tram for short journeys, but don't realise that in the place you're going, the opposite is true. Or whatever. It's the general idea I'm talking about, not any real-life specifics.
The other thing is, in your home country you can glance over all the places in a town or city that sell burgers, and be able to make a good guess, without even having to think about how you've worked it out, at which places will be pricey and high quality, which ones will be pricey for some other reason (location, extreme pretension, lack of other options), which places are ones you should go to for a reasonably-priced but filling no-frills family lunch, which ones would get a cheap burger in your hand within 40 seconds wrapped in paper, and so on. But if it's somewhere you don't live full-time, you might miss the little cues that tell you you're walking into the kind of place that'll charge you 50% more because they've got more adjectives on the menu and write their prices in a wanky way (like "14.9" when the convention is "£14.90").
On top of that, when you're away from home you might be making more expensive choices than you would at home anyway. More meals and drinks out, more travel on more expensive types of public transport, treating yourself a bit more, that kind of thing.
Like I said, I don't know whether England is more expensive than Ireland, it's just that it can be tricky to tell from a short visit to somewhere you don't live full-time.