As I understand it from my reading of Irish history, the Act of Union constitutionally merged the parliament of the Kingdom of Ireland, which had hitherto technically been independent (and had gotten too uppity), into that of the Kingdom of Great Britain, thereby creating the United Kingdom of Great Britain and Ireland, in much the same way as the 1707 Union of the English and Scottish Parliaments had created the Kingdom of Great Britain. As I pointed out previuosly, unless it's been changed, sovereignty still lies in the antiquated Parliament-plus-Crown set-up. Therefore, all subjects of His/Her Britannic Majesty in the UK were officially British subjects, which, for example, is presumably why the leaders of the Easter Uprising could be tried for wartime treason against the Crown, or why 19th-century coffin ships that America turned away could not be refused entry by Canada.
Whatever the truth of the matter, it wasn't really until the 20th century that ordinary folks ever started travelling in any great number and that people ever applied for a passport - and that increasing restrictions on international travel came into being. I just wonder what protocol authorities followed when dealing with 18-century Ulster-Scots escaping penalisation, or with 19th-century emigrants fleeing the Famine. If movies like 'The Godfather' are anything to go by, the "home" authorities seemed happy to get rid of you, and the American authorities gave you an austere but welcoming "come on on on in".
Hmmm. All this nationality business does complicate things today, doesn't it? That's why, when asked, I'm Irish when it suits me, and British when that suits me, too. But then, 1971 was the last time I ever lived in "Darkest Ulster" (as I then not very funnily called today's "Sunny Nornirn"), so what do I care! Nevertheless, on MArch 17th I be in our local "Irish bar", one of those identikit numbers sold all over that must be making a few boyos a euro or two. Slainte.