blabbermouth -- not precisely it, as this carries the sense of not keeping confidences.

gasbag -- still not precisely it, as this carries the sense of self-importance.

loquacious -- close but not it, as this can mean talkative without being irrelevant.

garrulous -- this might be it.

circumloqutious -- close, but carries the sense of talking all around a subject on purpose, whereas the irrelevant talker does not necessary seek to avoid direct speech.

Oops, the last three are not nouns. Oh, well.