Simple explanation of the Irish problem

With all the hassle over Brexit and the Irish border, I think it would be wise to bear in mind these two simple explanations of the British Isles, taken from 1066 and All That:

“The Scots (originally Irish, but by now Scotch) were at this time inhabiting Ireland, having driven the Irish (Picts) out of Scotland; while the Picts (originally Scots) were now Irish (living in brackets) and vice versa. It is essential to keep these distinctions clearly in mind (and verce visa).”

“Gladstone… spent his declining years trying to guess the answer to the Irish Question; unfortunately, whenever he was getting warm, the Irish secretly changed the Question.”

One explanation of the British Isles (there are many conflicting explanations). This one on MapPorn