Jack Gave a Complete Answer to One of Our Most Plaguing Questions: How He Came by His Stage Name
I was born in New York City and had three first names: John Joseph Patrick -- all saint's names -- as a prelude to my Irish moniker of Ryan. Years ago, long before I could possibly have anticipated playing McGarrett on the show, I set about finding a new name which would look good on a marquee, should I be lucky enough to ever get on one. I researched our family tree (my mother's name was O'Brien) and I found a branch named Lord; not the royal kind, but fishermen from County Cork. My nickname was Jack and I combined it with Lord for one reason only, I liked its punchy sound. Little did I dream back then that along with the rest of a struggling actor's problems, I'd wind up having to defend my own name because people can't accept my lifestyle.
Source: Borie, Marcia. The Jack Lord Nobody Knows. Good Housekeeping. October 1975.
Alas! The problems that arise when we have too much information!