Family line traced to Ireland but no Irish dna?

So I've been tracing my grandmothers family lines for over 5 years now and her paternal side is exclusively Irish for both lines up until as close as her grandmother who was born in Ireland in 1903. But she's done a dna test recently and has zero Irish. Along with two siblings who've done tests. But they all have almost 50% Scottish dna but no lines in Scotland.

I've got back to the mid 1700s and it's all northern Ireland on all direct branches. I'm trying to figure out if this is a horrific error we've made somewhere tracing the family lines for the last half a decade or if there was like a Scottish settlement in Northern Ireland that would have stayed generally separate from Ireland in the 1800/1900s? Lol. Thanks in advance for any help 😬