All the sources I've read say it's a Scottish/Irish myth - but the creature is said to come from the Highlands. The Highlands are in Scotland, so the creature would have a Scottish accent. It doesn't matter where the myth originates from - the creature in the myth still comes from the Scottish highlands.
Therefore, I wouldn't give him an Irish accent. If you must, give him a Scottish one, but please have a Scottish person look over it before it's completed so it isn't badly done.
Of course, you could just give him a made up accent. The Condie Petie mountain folk in FF9 are supposed to be Scottish and their accents aren't done too badly (written-wise) for some northern parts of Scotland.
Another thing to note is that this may be being referred to as "Scottish/Irish" mythology because of the confusion in the difference between the Scottish and Irish. It's humungous, but on the outside it wouldn't look like that if I'm being fair. I can tell the difference between someone from New Zealand and someone from Australia, and the difference between someone from the USA and someone From Canada, but some people might have trouble with that. I made the effort to learn so that I wouldn't offend anyone. From personal experience, one of the most annoying things is when people abroad (and usually only outside of Europe) say "Oh!, You're English!". They never say British. Or Scottish. Both of which are correct - English isn't. Say you live in the US and you're from California. It's like someone saying to you, "Oh! You're from Texas!" You'd be like... errr... no I'm not. The United Kingdom is actually "Great Britain" and "Northern Ireland" Great Britain being Scotland, England and Wales. They're kind of like states, except not. They used to be countries in their own right, until England... well, no need for a history lesson when you have Wikipedia.
Well, Northern Irish accents can be easily mistaken for one of those two, but if you're a native speaker of English you have to be pretty ignorant to get southern Irish accents mixed up with Scottish ones. It's only slightly less moronic than not being able to tell the difference between English and Australian accents (apparently some people can't).
Northern Irish accents sometimes get mixed up with ours because they're harsher than proper Irish accents. (For those that don't know (most likely only those not in Europe) Ireland is about 80% of the land it's on, Northern Ireland is the rest of it to the north. Ireland is not part of the United Kingdom. Northern Ireland is.) The southern Irish accents and much softer. Frankly, both are very sexual, so I'm not complaining.
English and Australian accents... I can't imagine getting those mixed up. However, I've always thought of the Australian accent as being a kind of strange mix of English and American. It sounds halfway between the two in my head. Again, I would say the New Zealand accent is slightly harsher.