You found me! I guess I'm the diser-Gráinne (unless there's another one out there!!)
I've only run into 2 or 3 of the same name growing up. And yes - it was mispronounced by everyone!! I always hated the first day of school or the sight of a supply teacher because I never know what they would call me. Of course, I went to school with a lot of Mary, John, Barb, Carol, Lisa type names so I really was the only one in those days. I think it's a lot more common now to have an uncommon or even just uniquely spelled name. I know I sometimes have a hard time with pronounciations when I volunteer with kids.
As far as how common/uncommon it is in Ireland...
It depends on where you live - it's definitely very popular in the South West. On my last trip there I visited the area it originated for the first time and everyone knew how to pronounce it - and said it so musically!
I have a lot of family on facebook (we're from the North of Ireland) and all my cousins seem to have at least a few friends with the name. Some with slight changes to the spelling - and their ages range from early twenties to late thirties - so it doesn't seem out of date there.
And the many ways I have to endure how it is pronounced....
grain, grainy, granny, grown, growny, grawn, grawny, .... I could go on and on.....
It can be annoying...but everyone remembers me! And when someone does pronounce it correctly they usually say - "I knew a Gráinne in highschool, dance class, church......"