You haven't thought much about the subject, then.

1. "The British" (whoever you imagine they are) do not want to remain in Ireland. Regrettably, it is impossible to withdraw the token military presence in Ulster until the Unionists are no longer a potential threat to the Catholic minority; and, indeed, until all paramilitaries of whatever pursuasion agree to abide by rules of behaviour that apply equally to all parties. This is likely to take at least another generation. It will probably take longer than that for the Irish government to take the steps necessary to ensure that re-unification (which must take place eventually) will actually work.

2. Hong Kong was leased. The lease expired. The tenants left. Simple; though, to give them due credit, the departing tenants insisted on guarantees of basic rights, not allowed in mainland China, for the remaining inhabitants. Comparison is irrelevant.

When I was a naïve teenager, I believed that the British government should withdraw immediately. I changed my mind after meeting a Catholic peace activist who explained what was really happening there. Like it or not, a military presence was needed in the 1960s, and is needed now, though to a lesser extent as the various parties begin to grow up a bit. That military presence, however, needs to be perceived as neutral, and due to the kind of incompetence that led to disasters like Bloody Sunday, it's perfectly true that that is unlikely to be achieved under present arrangements; a United Nations peace-keeping force might be a good idea, but it would need to be very highly trained. Whether or not any of the various factions involved would be prepared to accept that I do not know.

The thread title is, frankly, bloody stupid and calculated to cause dissent. Consider, for example, the occupation of the North of America by European settlers; or, looking further back, the occupation of the West of Scotland by invaders from the island now called Ireland. Are they illegal too? If the issues cannot be addressed dispassionately and without partisanship or bias, they will never be resolved.