Words such as irregardless and theirselves are errors, shunned by language gurus. Avoid them. The correct versions are regardless and themselves.
When you are not certain whether or not a word exists, consult a good, full-size dictionary. (Pocket dictionaries do not list enough words to be consistently reliable sources of information on the existence or nonexistence of a word.) Many careful editors, even those who have an excellent vocabulary, frequently consult a dictionary.