There are two fallacies here at the same time.
First, taking the premise "Mr Smith is clearly irrational about some things" and concluding "Mr Smith is irrational about everything".
Second, taking the premise "Mr Smith is irrational about something" and concluding "Mr Smith's claims about something must be wrong". If Mr Smith is irrational about something, then his opinion will be totally unrelated to the facts, not the opposite of the facts.
(Plus the minor detail that Mr Smith might be trying to mislead you, and what he claims might be the opposite of what he believes. And since he is sometimes irrational, that's not too unlikely).