We know that

1. Cain and his descendants most likely did not survive the Great Flood. So it's pretty weird that someone would be his "son" thousands of years after the Deluge.

2. Even the same pronunciation is often renderred differently across different languages. So it's unlikely that the Gaelic (a Celtic language) would spell "Cain" exactly same way the Anglo-Saxon (a Germantic language) translators of the bible spell "Cain".

3. Cain, as a word, is Hebrew, or at least Semitic in origin. Moreover, in its original form, it is written as QYN, with the pronounciation "Qayin" being inferred from tradition. In other words, Cain's name may or may not have been pronounced appropriately. Therefore you cannot infer that Cain in Gaelic (an Indo-European language) means exactly the same thing in Semitic.

4. The Scots and the Irish are Christian converts. Imagine anyone claiming to be the descendants of Cain for 1,000 years of Christian rule. If the Jews were persecuted despite numerous warnings from the Catholic Church against antisemitism, what do you think might happen to the descendants of Cain? (hint: the monster in Beowulf, Grendel, is said to be a descendant of Cain in the epic poem.)

Wikipedia can be edited by anyone, as long as they have a credible source, so in and of itself, Wikipedia cannot be said to be credible without another verifiable source. If you check the entry on McCain now, you'll see that I've already corrected it. (Yes, I edit wiki sometimes.)