I have spent a year and one half tracing, back and forth, and looking through many search engines, my Famly Ancestry with the following results:
England: From Alfred the Great (849) forward to William the Conquerer (1027)to Robert I (the Bruce of Scotland) thru the Kennedy Clan of Scotland to 1696 James Cassels (a Sept) to myself Bertrand B Cassels Jr. (1937)
Scotland: From Malcolm III (1037) of Scotland back as far as Alpin of Kintyre (UNK) King of Scotland.
Ireland: (Kennedy -- O'Cinnedie -- Cinnediedigh to Brian Boru King of Ireland (926) and beyond in most cases to the 6th Century.

I am sure that many Cassels Branches can do the same by Starting with Bertrand B. Cassels Jr, Brian Boru, Robert the Bruce, Alfred the Great, David Cassels (1819) my ggfather, Robert III of Scotland, Mary Stewert (Princess of Scotland, Saint Margaret of Scotland and William the Conquerer.

Good luck

Bert Cassels