I think many Brits implicitly assume that Irish people are Brits too who are suffering from false consciousness, since everyone here is inflicted with "irrational" nationalism as a result of fuck ups from the 19th and early 20th centuries, but given enough time, they will come around.
The big problem is that you are working against the headwinds of 500 years of history. Every time British people mention something "we did together" or try to make light of events from whenever, it risks making you look like an apologist for your past selves by pushing a rose-tinted perspective. And if you aren't willing to condemn your past selves, why should someone trust the UK not to have irredentist sentiments now?
Additionally, this isn't going to go away in the future, because the UK is in decline at the moment, to put it very very bluntly and very very harshly. So after considering the place beneath notice for seven decades, in the past few years the UK has suddenly decided that Irish were valued BFFs all along and we need to rejoin (after previously being secretly relieved to see the back of us).
The problem is that any kind of "near abroad" policy tends to generate tensions. People can intuitively sense when someone has designs on their country and it naturally tends to irritate them. When there is previous bad history, this feeling will manifest itself in the form of nationalism.