I agree with Whistle Stop, it will depend on a number of factors.

1 - Are you comfortable with the information which you will impart
2 - How is the venue and audience appearing
3 - Do you need time to tune?

As a rule of thumb, you should keep the introduction shorter than the song itself. Try not to digress too far in telling the story.
I would believe that in a bar situation it's touchy to spend too much time talking about the song. In a concert type venue or a song circle, it would probably be common to explain a little about the song.