Well, Ireland was in the UK until 1931; the Irish Free State had dominion status until then and commonwealth status until 1937; recognition of the English monarch in any state matters ended in 1948.
And the irony is that they were “united” with Britain again under the EU.
That's like saying that Finland and Spain are somehow one nation because they are both in the EU.
The point is, a bloody war was fought for Republic of Ireland's freedom from UK, harking back to the horrors of the potato famine and the political oppression of Irish Catholics, and they felt justifiably disrespected.