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As best I understand the road numbering system here are the differences:

National Primary Routes "A" roads - nice divided highways. There are not many of them.
National Secondary Routes "B" roads - two lanes, stripe down the middle, a stripe on both sides.
Regional Roads "A" - stripe down the middle
Regional Roads "B" - say three Hail Marys and start driving!


Don't worry about getting lost in Ireland because there are signs pointing the way. One small problem is that the signs don't appear until it's time to turn!!!

 Yep, both of these signposts are on the same corner!