Do you have any evidence to support your claim?
I doubt it – that you have evidence, that is. You can’t even get your dates right.
- There was no “Britain” or “UK” in the 17th century.
- There was no UK which included Ireland in either the 17th or 18th century.
- Irish Catholics were forbidden from joining the British Armed Forces throughout much of the period until the Papists Act 1778, and then only on condition of swearing an oath abjuring aspects of Catholic doctrine.
- The Duke of Wellington was an Anglo-Irish Protestant, active in the late 18th and early 19th century. Irish Protestants were not prevented from joining the British Armed Forces.
Ireland was a Lordship of England from 1171, a status created by Pope Adrian VI’s Bull, Laudabiliter, in 1155 and later confirmed by his successor, Pope Alexander III. It remained a Lordship of the English Crown until Henry VIII, King of England (note: still no Britain or UK), converted it to the Kingdom of Ireland, held with the Kingdom of England in “personal communion” by the monarch.
When James I (of England ) and VI (of Scotland) ascended the English throne, he held the 3 Crowns of Scotland, England, and Ireland in personal communion. This state of affairs continued until, during the reign of Queen Anne, the Kingdoms of Scotland and England were combined by the Acts of Union 1707 into the Kingdom of Great Britain (now there’s a Britain and a UK, but one that doesn’t include Ireland). Thereafter, her heirs and successors held the 2 Crowns of Great Britain and Ireland in a personal communion, until 1800 (19th century), when, under the Acts of Union, the separate kingdoms of Great Britain and of Ireland were combined into a single Kingdom of Great Britain and Ireland (a UK including Ireland).
Once Irish Catholics could join the British Army, they did so in fairly large numbers; then again, they also joined other professions in increasing numbers, as they became open to Catholics.
I think it very unlikely that any Irish people claimed to be British, except where legally required to do so. Certainly, looking at military and immigration records for my ancestors, where a place of birth was asked for, the answer was “Ireland”, not the UK; and where citizenship (UK) was required, there was usually a second question about nationality, where they answered “Irish”. NB: most of my ancestors were Irish Protestants – it was not unusual for Irish Protestants (such as the Duke of Wellington) to claim to be both Irish and British and to think no more of it than a US citizen would of calling themselves both American and Oklahoman. My ancestors appear to have considered themselves more Irish than British, however, and I can’t think of a reason why Irish Catholics would not have considered themselves similarly, the legal niceties be damned.