[worren]

As I was later to understand, "The Republic" is slang that indicates a sympathy for Republicans, Irish Catholics, some of whom rather notoriously caused "trouble" for the Crown and it's subjects. The offences were mutual. Open hostility relatively rare now, but there is still a lot of animosity and caution reserved for suspected troublemakers of any stripe. As an American, I am certain I was treated deferentially. I had, in effect, just given the agent the finger.

[scoot]

Leaving aside your rather ill-informed history lesson - indicates to whom?

As a British born non-catholic who has lived and worked in both Ireland and the UK, I can assure you are over-stating its meaning, except perhaps to practicing loyalists in Northern Ireland. The accepted vernacular there as a matter of practical sensitivity to the majority is "Southern Ireland" or "The South" and "GB" for the part of the UK that excludes NI.

To suggest that it is generally offensive (as in "giving the finger") is, well, offensive.

Edit: To wit - "UK Border Agency | If you are travelling to or from the Republic of Ireland [...]" http://www.ukba.homeoffice.gov.uk/customs-travel/Enteringthe...