From the "Historical Research Center" I have the following "Family Name History"
"Surnames are not just words or sounds. They originated as descriptions of the person for reasons of better and easier identification. These early additional names were discarded with each succeding generation. They described one individual and not his whole family. These names dated from the early Middle Ages. With regard to the Scottish surname MacKeachan this name is of patronymic origin. Patronymic surnames are those names which derive their origin from the personal name of the father of the initial bearer. In this instance, the surname Mackeachan, along with varients including Macceachan, Maceachain, Maceachen, Maceachin, Maceachan, Macechan, etc., is the anglicized form of the Gaelic surname Mac Eachainn which means literally 'son of Eachan'. The personal name Eachan is in turn derived from the Old Irish (the origin of Scots Gaelic) Eachdonn (attested in 1092) from ancient Ego-donno-s meaning 'horse lord'.
It has been said by some of the Mackeachans that they are in fact MacDonalds while Allan R. MacDonald claims they are really MacLeans. Indeed they are listed as a sept of the MacDonalds of Clanranald who claim descent from Ranald father of Donald. One of the earliest records of the surname is a reference to one Gillecrest Macachin who witnessed a charter by Roger de Scallebroc of land sin Carrik in the reign of William the Lion. Andrew McCachin was rector of Ardmuch in 1506 while Neill Makachyn and Malcolme Makachyn were killed in 1508. Ewan McEachan was a tenant under Chisholm of Eerchles in 1721. Many of the Mackeachan sept settled in the Canadian regions of Nova Scotia and Prince Edward Island where the surname is found today in quite large numbers."