"meaningless/irrelevant" is the adjectival form of moot, not the verb, and it's worth reinforcing that this is a usage shift specific to North America. To "moot it" doesn't mean "to make it irrelevant"; rather, things originally became "moot" when they were topics you could argue over but which had no further relevance in the law, and the latter grew to dominate its usage in the adjectival form in North America. But that usage didn't migrate back to the verb, or appear at all abroad.
Here's a usage in verbal noun form to mean "a meeting for discussion": http://www.themagicboxstore.com/paganmoot.htm
And there are, of course, the well known "moot courts", so named not because they're "unpractical/irrelevant/meaningless" court but instead because they're forums for the students to debate the law.
Common? Please can you point to any online usage of the word in that way?
I know lots of people use it to mean "no longer relevant".
> 2. N. Amer. (orig. Law). Of a case, issue, etc.: having no practical significance or relevance; abstract, academic. Now the usual sense in North America.