An almost invisible faultline

The drive from County Mayo where Patrick's mountain stands up to the ferry that took us to Scotland took is across the boundary between Ireland and North Ireland, from the Republic to the Kingdom.  I had allowed for some time to cross the border.  I had heard stories of the Protestant north vs. the Catholic south, and we all know of the atrocities committed on either side.  Such a place of tension would certainly be marked by armed guards, razor wire and menacing looking gates.
The only indication I had that we had crossed the border was a simple and small sign that said, "Speed limits marked in miles per hour."

What is the significance?  Ireland is part of the European Union.  The currency is the Euro and the speeds are marked in kilometers.  Signs are all printed in Irish Gaelic as well as English.  The UK circulates the pound sterling and marks distances and speeds in miles.  Only in Scotland and Wales are there bilingual signs.  It came clear to me an hour up the road when we were stopped by a "police" (not a "garda" of Ireland,) because a short parade was moving through the center of the town, flying the Union Jack and featuring kilted pipers.  We were definitely no longer in Ireland, we were in the UK. It seemed simple enough, but there is no mistaking the boundary. It marks an almost invisible faultline.

How quickly we bury our deep divisions with commonalities, only to have them emerge once more as hidden and dangerous faultlines.  How much better to name them, own them, and learn to value the other precisely for their differences.