The Northern Irish connection is quite interesting. A noted Gaelic scholar, William Matheson, formulated a theory that the Morrisons of Lewis, or at least, the direct chiefly line, originated in the area of County Donegal. He argued that they migrated from North West Ireland to Lewis sometime in either the Twelfth or Thirteenth Century. Matheson talked about the close connection the Morrison chiefs, as legal advisors to the Lords of the Isles, maintained with the premier Gaelic legal family in Ireland, the Mac Aodhgans (McKeegans). Apparently MacKeegan, was used as a surname in Northern Lewis in the Eighteenth Century, although it has since vanished from use.
Might it be possible that your Morrison line had ancient origins in Ireland before they moved North to the Hebrides?