I would say if every song "needs" to be introduced, a new mix of songs is called for (or a less anal-retentive attitude about it). Many songs speak for themselves quite well.

Others do benefit from some background info, depending on the setting. When I sing "Come out ye Blacks and Tans", I usually say a few sentences about who the Blacks and Tans are (were?), how most Irish folks seem to regard them, and how the songwriter's father inspired the song by coming home drunk and calling out his neighbors, many of whom were retired British Army people. I think that helps them understand and appreciate what is being said, which isn't self-explanatory to those unfamiliar with the history of the situation. But if the crowd is one that is pretty familiar with this branch of Irish music, there's no need for an intro.

I like to talk briefly about a number of songs--emphasis on briefly. I'll introduce "Matthew" as a John Denver song that hasn't been played to death. I'll introduce "Paradise" as lamenting the coal mining coming to an area, followed by "The L&N Don't Stop Here Anymore" as lamenting coal mining leaving. I think most songs would suffer from a long lead-in talk.

It has been pointed out to me that most people don't give a rat's ass who wrote the song. Mention the songwriter only if there's a specific reason to do so.