The Black Watch tartan is so common and so widely used by the garment industry in general that I wouldn't think it would matter. What do you mean about wearing the Black Watch tartan in Ireland? When I lived there I wore my Black Watch all the time! (It is a Canadian Army surplus tank) No one seemed to care!
I would worry more about wearing a tartan baseball hat with souvenir pins, some sort of Scotland T-shirt, shorts, and a loud, nasal voice asking where the "Bathroom" is! (Anyone remember Trainspotting?)
When I lived in Ireland I used to dread the tourist season. Throngs of old, well-off North Americans would comb the countryside in search of leprechauns, shillelaghs, and little stuffed donkeys! All the while, asking every local to repeat him- or herself! "I just can't understand a word you're saying!" I used to always feel mildly embarrassed but you can't blame them - blame the media!

Is mise le meas,

Seán Liosliath Ó hAirt