I've never heard of this one, but when I was in the forces, Catholics were Grapes, and Protestants were Oranges, Protestants after William of Orange, but never did find out about Grapes, i'll keep an eye on this, as both may be answered.
It is a derogotory term that is believed to date back to the 1920's Catholic Glasgow street gang the Tim Malloys. The other theory is it is an english translation of the term Taig was used as a common name for Irish immigrants to Glasgow.
I'd say that Celtic fans (who are often assumed to be Catholics) are referred to as tims. Catholics across the globe certainly aren't referred to as tims.
See Thewiseman's first answer for an explanation of this Celtic/Glasgow theory.
Also, given that Celtic fans refer to themselves and each other as tims, I'm not 100% convinced that it's exclusively derogatory. I've heard many other derogatory terms and people who use those don't tend to say "tims". Plastic Paddy, Pape etc. yes. Tim, no.