Celtic culture had tribes all over Europe. They are an ancient people and/or their desendants, and some people who choose to follow Celtic Paganism also sometimes refer to themselves as Celtic. The word celtic could also be the language, the artwork, the spirituality,etc.
They were refered to as "the people from out of the darkness", because people knew nor understood anything of their language, culture, or spiritual beliefs and traditions. Much less where they came from, only that it was from somewhere out of Ireland.
Some celtic traditions live on today with a bit of variation from the original rituals, such as Halloween, which was Samain.
Does mean wizzard or sorcerer but it also means demon. Broken down however, judging from it's roots the word is meant to have negative connotations. The word comes from waerloga- meaning a traitor, foe, devil. Broken down< waer= a covenant+ leogen= lie, deny. So you see it literally means to lie to or deny your coven. Any self respecting pagan would not refer to a basic male practitioner as a warlock, unless it was meant as an insult. Or used to describe one who operates on the negative side of their practices.
It was determined that a warlock put a curse on Cavan's family, destroying more than half of his lineage.