Sure, the populations of both California and Texas voted to accept admission to the United States and were accepted.
Another difference is that California and Texas are not in open rebellion against the government. THEIR populations WANT to be in the United States. Pretty big difference, eh?
Really? Like Northern Ireland really belongs to Great Britain? Or the Sinai or the Golan Heights really belongs to Israel? Or like Kuwait should have belonged to Iraq?
The population of NI voted to become part of the UK, and gets a vote at least every 5 years. Every time they have voted to remain part of the UK.
Northern Ireland is not in open rebellion against the government. Around 10% of the population supported Sinn Fein/IRA, the other 90% were opposed to terrorism. A majority of the population, way over 60%, want to remain part of the UK.
NI was settled by Scots and English hundreds of years ago, long before Texas and California were settled by Europeans. The Sinai was conquered by Israel 50 years ago, the Golan just over 30 years, Kuwait ten years.
None of these are comparable to NI.
Ireland shouldn't have been invaded 800 years ago. However, what happened 800 years ago was the norm for Europe at the time. Now, NI is a part of the UK, with full democracy, inhabited by people, most of whom want to remain part of the UK. There is a small terrorist problem, perpetrated by groups who are opposed to the democratic process, and want to impose their will on the majority.
NI has more right to be part of the UK than Texas and California do to be part of the US.