Although it has come to sound Irish, Dennis is actually of French derivation (St. Denis is the patron saint of France) and harks back even further to Dionysius, the Greek god of wine and debauchery. It was introduced to England by the Normans.
Dennis was considered cool when it was near the top of the charts in the 1940s and 1950s, but has been slipping ever since -- now at Number 496, thanks in part to the devilish Dennis the Menace.
Dennis Barlow is the main character in Evelyn Waugh's satirical 1948 novel The Loved One. Some prominent namesakes also include Dennis Hopper, Dennis Rodman, Dennis Quaid, Dennis Leary and Dennis Wilson. Denis is a secondary spelling.