I have heard several “Black Irish” Theories with the Spanish Sailor theory being the most common. Google “Black Irish” and you can see that there are many different explanations, but none have been proven. And some claim there is no proof the Spanish sailors breed with enough Irish to have the affect they claim it did. In any case I don’t know if my clan was in Ireland long enough to intermix with Spanish/Irish. We were Scots-Irish as were many of the people in Northern Ireland (Ulster). My family moved from Scotland to Ireland in the 1500’s and then to America about 1750. Most Scots-Irish did not even interbreed with the pure Irish on the island, keeping the bloodline pure Scot for generations. So even though my GGGG Grandfather was born in Londonderry and his family live in Ireland for about 200 years they were probably pure Scot.
It is possible the Celts were not quite as light as the Teutonic people. The Irish Celts may have come from the area of northern Spain. Later Ireland was almost entirely a Niorse colony.
It difficult making genralizations about European ethnic groups due to the tremendous mixing and movement of peoples throughout history.
Look at Italy - Latins, Goths and Vandals from Scandinavia, Spaniards, Moors - all mixed together.
Or England - Celtic Britons, Irish, Picts, FLemings, Anglo-Saxons, Norsemen and French Normans.
That is a pretty good stretch, with some wishful thinking, considering you are talking about 4, possibly 5 generations of breeding in a foreign land with foreign people. Considering the size of families centuries ago, it's not just unthinkable, but probable, that someone, or several someones, wandered into the local population. Ireland is not a big place, if you've ever been there. A young man in Ulster could have found farm work in the central plains or work on a harbor in the south. Then again, some lone Spaniard could have wandered into camp in Scotland and these dark traits were introduced prior to the emigration to Ireland.
Since you seem willing to dismiss that your bloodline is pure, then the only other explanation is that you believe that "dark" people actually inhabited places as far north as Scotland. I've never known any dark Scots or Irish except my own family which is descended from the black Irish. Most are fair with blue eyes and either brown, red or blonde hair. Dark skin, brown eyes and black hair would certainly indicate a gene introduced from elsewhere.
Many, probably most Scots Irish descend from Scots who themselves descend from Irish and Viking forebears who met and mixed in the 900s.