My husband is a Cassels from Ayrshire. There is some connection with the Kennedy family of Culzean, in Scotland, but it is most likely that my husband's forebears, like many others, took Cassels/Cassils/Cassells etc as a surname when these became more usual, bacause they worked on the Kennedy estate rather than because they were related. Kennedy is the family name of the Earl of Cassillis/ Marquis of Ailsa. The Earldom was created in the 16th century - Cassels in any form was not the family name of the Kennedys at this time.

Elsewhere especially in England and Ireland there are Cassels families unrelated to the Kennedys.