Slightly OT, but there is no such place as "Eire"! It is the Republic of Ireland or various derivations, like "Irish Republic". "Eire" is on the country's stamps. It's like calling Germany "Deutschland". Technically you can walk across the border from Northern Ireland into the Republic and vice versa. And locals do. You can cross on many small rural country roads. Some of these crossings are used for smuggling (of fuel, cigarettes etc) and other nefarious activities. Also walking across the frontier on one of those roads could also mean you make the acquaintance of heavily armed police officers on either side of the border . These will not be your average "beat" officers but counter-terrorism or undercover/the equivalent of plainclothes SWAT officers (if you want a US reference point). And these officers would have lots of questions for you -- rather more than an immigration officer .