Obviously we can only guess at the tuning they used.
Some tune to a pentatonic scale. I use the first six notes of a standard major scale, in the case of my current instruments, GABCDE. This gives me access to G major and A minor (modal) with the possibility of modulation to C major (with an implied third in the dominant.)
As with any string instrument, tunings can be adjusted to suit your repertoire (although efficient tuning pegs are still evading me - my lyres are a bit of a b***** to tune!)