Well, it might be irrelevant to me, but it's probably not to them. So I just listen and try to muster up all the appreciation for their subject as I can :)
I say :
"(insert irrelevant persons name here) I don't care"
"What the **** are you talking about?".
"What relevance does this have to XYZ?"
"You do know I charge by the minute, right?"
Or something else on the spot. I generally just get them speaking super fast through body language techniques, so get it over and done with quickly.
Sometimes I also do the same to them (who are talking irrelevantly). That's the only way I can do, right? k.
It's not good to deal with people like that because it makes us look bad.