Scottish? Irish? Scots-Irish?

Scottish?
Irish?
Scots-Irish?

Within sight of each other on a clear day, Scotland and Ireland are also close in ancestry, heritage and culture through a common Celtic history. Scotland is actually named after an ancient Irish tribe, the "Scotti" of Dal Riada who migrated to Scotland starting circa 400AD.

Surname spellings create confusion as to national origin, in particular the myth that the "Mc" and "Mac" prefixes indicate nationality (Irish and Scottish respectively) in particular.

Many Americans have Scots-Irish lineage and are unaware of it despite this being the second largest European immigrant group before the American Revolution.