Here is a history.

The tenor is usually played with a plectrum. There are 2 main tunings "Jazz" or Standard which is CGDA and "Irish" which is GDAE. In jazz, the instrument is used for chordal accompanyment but in Irish and I think most other folk, it is nearly always used as a melody instrument.

Sully produces a tutor book for the Irish style and I believe Gerry O'Conner also does one.

Jon