It's probably either based on OS/Browser regional/language settings or by tracing the country your IP address resides in.
If using Windows XP, try going to Control Panel >> Regional & Language Options. On the first tab there is dropdown for Location, probably set to 'Ireland'. Set to UK and restart machine.
If based on IP, there are sites that will anonymise your IP basically by acting as a proxy which might work (can't think of the name right now though, try searching).
You might also clear your cookies (or cookies for this site) as they may have cached something the first time it re-directed for future performance.