Greg
MacLysaght derives the Irish MacGahan and Gahon from Eachain
Kneen derived the Manx Kaighin from Eachan, a horseman; Thomson gets it from Eachunn - whatever the difference, defining it as a "traditional Gaelic personal name" - and Moore from Eachan saying it means a knight or horseman.
The fact that we have never heard of the forename Eachan or Eachunn reinforces how long ago the Manx surnames must have formed, which is the thrust of Prof. Thomson's article. He thinks they may have become fixed by 1376.
How do you see the derivation of your name?