You could pretty much state that Ireland and its various parts have been patrolled by armed units from Britain for the past 700 years.
But Northern Ireland is not, and has never been part of the British Isles! It is part of the United Kingdom but was never part of Britain.
Or to put it another way Ireland is occupied by the Brits which of course explains why troops had to patrol Northern Ireland. If you read my comments you will see I explicitly referred to Northern Ireland.
You are right I should have written Great Britain / British Mainland. The nuances of politics on the British Isles are exasperating. I am acutely aware that Army in Northern Ireland operated and operates very differently from the rest of the UK.