> Ireland dislikes the UK since the UK invaded it

This was centuries before the UK. The Normans came to Ireland by invitation of Macmurphy, King of Leinster, to help him restore his power, in exchange for promises of territory. This barely counts as a conquest CB, but with certainty not as an invasion.

> The open border treaty was put in place because the alternative was either giving the territory back, or nonstop terrorism (look up the Irish Republican Army) until they gave the territory back.

The Common Travel Area's origins are in the the period 1923-1925[0], although it wasn't called that back then...

[0] https://en.wikipedia.org/wiki/Common_Travel_Area

this is a genuinely awful description of Irish history

Speak for yourself, not the Irish.