See the book on my website.
Lismore Isle became a point of contention between the two clans in the late 13th century. Alexander Macdonald, heir to the Lordship, fell in love with and married Juliana MacDougall. Her dowry was to be Lismore Isle where Castle Coeffin then stood. But Alexander MacDougall, the MacDougall clan chief, reneged on his promise and denied the dowry.
Ultimately, King Edward I of England would decide for the Macdonalds. You'll be able to read about it in the story I am writing now, Bound by Honor, coming later this year.
See a description.