"Session" tunes are dance tunes; commonly, two or three tunes are linked in sequence, one after the other. These are tunes passed aurally from player to player over hundreds of years. The transcriptions are only interpretations of the tunes; many players embellish the tunes as they play. The chords are only suggestions -- you may wish to alter the chords as you play. Note that most tunes follow the traditional "I-IV-V" chord model.
You may also adjust the "tempo" of these tunes; many sound great when played quickly with spirit. Others are best when played slowly with feeling. Experiment and see what works for you. You can use the provided MIDI files to help you determine the proper tempo (many midi players let you adjust the speed).