I seem to recall the reason for Gareth's assertion that the British & Irish drive/ride on the left was that whilst travelling, historically one's shield would be on the left arm providing protection against ambush from the nearside of the road.

Anyone attacking from the 'offside' would lose the element of surprise since they had further to travel to cross the carriageway before reaching the traveller.

Similarly, whilst jousting the knights would meet shield to shield with their lances aimed over the left shoulder of the horse aimed at the opponents shield area. i.e. each would pass to the right of their opponent.

Bob vG