Many people believe a "Webster's" dictionary is authoritative.
However, there is neither a copyright on the name nor a set of
standards for publishing a "Webster's" dictionary. Therefore, anyone
can publish his own version of a "Webster's" dictionary without meeting
any generally agreed-upon definitions.
Additionally, Webster's dictionaries tend to be "descriptive" - that
is, describing how words are being used in society, rather than how
they *should* be used.
The more prescriptive American Heritage College Dictionary, Third
Edition, provides these guidelines on "irregardless:"
ir-re-gard-less adv. NON-STANDARD. Regardless
Usage note: The label Non-Standard does only approximate justice to the
status of irregardless. More precisely, it is a form that many people
mistakenly believe to be a correct usage in formal style but that in
fact has no legitimate antecedents in either standard or nonstandard
varieties. The word was likely coined from a blend of irrespective and
[with apologies for continuing OT]