Clan MacDougal originated with a son of King Hakkon the first. The clan coat of arms shows longships and I read somewhere that the clan could put several thousand men at sea at one time.
http://www.clanmacalistersociety.org/historians-corner/article1.htm
This clan was from the SW corner of Scotland (Ayr) and sailed amongst the islands mentioned previously. The distance from the Scottish Isles to Northern Ireland is less than 20 miles, so intermarriage would certainly happen.
According to the history above, the King of Norway controlled these islands up to the 12th Century.
I'm proud to be from a smaller Clan within the Macalisters...:^)