We cross the Irish-Irish border (?) without knowing it. No checkpoint, no signages, nothing, not even a pothole or a hump on the road, just a harmless notice saying County Armagh. It happens soon after passing through the town of Dundalk, in County Louth (Rep. of Ireland). Well, maybe the clues are the odd 'BRITS OUT!' graffitis.
We only realise we are in Northern Ireland when the milestone numbers suddenly become smaller (UK uses miles) and advertisements in £ instead of €. And of course petrol at almost £1 a litre - being pro-active we filled up our car in Dublin at just €1 a litre. :-)
Soon we enter Belfast, infamous for The Troubles. But that's all in the past ... I hope.