Aniar aduaigh can also mean 'out of the blue', I believe. 'Thainig sé aniar aduaigh' - It came out of the blue/as a surprise.
You're right. The history behind that is the north-west is supposedly the only directiom an invasion hasn't occured from. So if it comes out of the north-west, it's a surprise.
Nobody expects the Icelandic Inquisition! ;*)
eða gera þeir ...
Maith thú, I had wondered about its history. I had guessed maybe the remoteness of Donegal but your explanation makes sense.
Can ag teacht also be written as ag teach ?