Severely religious people...
Sure, have faith! That's a great thing! BUT DON'T GO KNOCKING ON MY DAMN DOOR AND TRY TO CONVERT ME!!

People who insult politicians that don't know anything about politics. ARGH! I don't know that much but I at least have the decency not to whine about them when I don't even know what they do properly!

And last, it's just getting a bit too much but not necessarily irritating- people who argue over the creation vs evolution... mainly focused on religion vs science to be honest. I am tired of people arguing over this any time someone mentions something about god/evolution. Please go take your argument somewhere else.