As for validity of IR for those who live elsewhere:
Yes, its indeed country of RESIdence, for at least 6 monthes, with proof of that from an official institute (and other as in The UK about any EU country wants you to register at a local council after 3 monthes or so of living there permanently-the time you can visit without any other burocracy, and usually a slip or note for that is given out-in most countries you are also supposed to de-register when leaving, which many forget to do, the registration also makes you a victim for charging local taxes). When buying an IR (it is still all based on the old style procedure of getting i over the counter at an appropriate office) this country is checked and should be written in the travel report-, which also has a special box for 2 trips to be noted for getting Out+in in that country. In actual practice this is very, very rarely checked. It thus remains a bit of gray area. Its even more prone to fraud for f.e. those from USA who can order an EUrail which is now nearly the same as IR and at same pricing, but has not the only 1x Out+In rule on it.
Now try a cheap 4/5-day IR or for that matter a 2-month IR to cover say Inverness (yes, Thurso/Wick is even further out) to LGW=LOndon Gatwick as this is also allowed, and compare with your local skyhigh exBR-fares for that. Free train choice too.