Richard, if you are near a poker room, pick up Poker Digest. I have an article in about Ky illegals. This issue should be in the rooms for another week.
John gave you a pretty good description of the term Illegals.
Most illegals closed in the 50's and early 60's. They were all over the country, Northern Ky, Galveston, Hot springs, Miami, and several cities in Ohio. I am sure I left other spots out, but should give you a general idea.