If you look at the IRB site, you'll see that it has about 100 ranked member unions. Pretty much all of them will have participated in the qualification process, with the exception of a few recently organized Unions. If a nation isn't in that list, then it did not compete. See
If you mean any countries that are not in the final group stages in 2015, there are many. If you mean those that don't compete to be a part of RWC, much less. As an example teams Samuel listed like Brazil, Spain, Mexico, Uruguay all competed to be a part of the RWC. The Irish team encompasses Eire and Northern Ireland.
