You are probably in the same boat as me. My surname Donabie is also connected to the Clan Donnachaidh (Robertson), but may have evolved from Donachie or Dunachie or Dobbie. Your last name Tonnachy is in the same catagory Donachie, Donachy, Conachy, Connachy, Connochy, and may have been changed after the battle of Culloden to avoid persecution by the English.
Have you ever thought of Y-DNA testing to verify a Clan connection or find other decendants?
I am participating in the Clan Donnachaidh DNA Project and
have signed up to Rootsweb Clan & Decendants Project.
Perhaps others who are direct relatives to you but have similar but different last names will find you through this process.
(Donaby, Dunaby, Danabie)