I can never figure out if it's ignorance of history or deliberate obtuseness that leads English people (Scots and Welsh seem to get it, IME) to a lack of understanding of what Ireland is.
Perhaps they are equally ignorant of other countries.
Ireland, the island was a single country prior to being invaded, first by the Normans, then subsequently England.
Until 1801, Ireland continued to have it's own parliament in Dublin, but after that, was ruled directly from London.
Over the entire period that England ruled Ireland, there was gross, derogatory discrimination against the native population, destruction of the language and heritage, removal of natives from their land and placement with emigrants from Britain; mismanagement leading to major famine and emigration, lack of education for the native population, lack of industrial or agricultural development.
The London government, insisted on labelling Irish people as backward (no opportunity for education -the majority Catholic population could not access education unless wealthy enough to send children to France/Spain for education) dirty (no nationwide road or sanitation network) stupid (cf education) etc etc.
All of which was very handy, for propagating a sense of "otherness" about Irish people. And, as a majority Catholic country at the time, Ireland had to be subjugated to mitigate against the "back-door" threat of invasion into Britain, from France or Spain.
All of which left Ireland, at the edge of Europe, but considerably poor and undeveloped.
After 1916-21 (rebellion, war of independence) Ireland finally succeeded in achieving independent Dublin based government, and the country could finally start to determine it's own path. At that time, Lloyd George (British Prime Minister) insisted that Britain would retain 6 counties in Ulster, which is now Northern Ireland.
That portion of Ireland continued to be mismanaged; and the Catholic population there continued to be discriminated against. The Catholic population co-opted the approach of black people in the US to protest against that discrimination- which is what kick-started the "Troubles".
The "troubles"/civil war in Northern Ireland was entirely a conflict inside the borders of the United Kingdom; and technically nothing to do with the Republic of Ireland.
The attitudes toward Irish people which were fostered to subjugate the country over centuries, have persisted into the present day.
I've been asked told by an English person (from Nottingham) this century "don't we still own you"?
I mean, who says that?
Don't I still own you?
I happen to like English people and the country and it's history and it's achievements; and there are many things that the world is grateful to Britain for; but the ignorance shown, would be worthy of an AIBU thread if we were two houses adjacent to each other.
So, yes, OP, you are not unreasonable to go on and on and on and on, reminding British people that Ireland is a separate country which is equally as separate as France, Norway, Denmark, Germany, Hungary, Brazil, Laos etc.