Why do you think UK media repeatedly claim Irish victors in sports and entertainment as British?

Given a lot of Irish stars have to work in British film and TV too for exposure and many actually build their career here, they can be a British icon at the same time as being Irish, and I guess that can get confusing. For example Cillian Murphy's breakout role was 28 Days Later, a British film; one of his most iconic roles is playing a Brummie in Peaky Blinders, so despite being Irish he is also an icon of British cinema/TV. Another example is someone like Graham Norton who obviously if we thought about it, we'd know was Irish, but whose work is so effectively British we might forget that at tmes.