That is completely cool! You must have seen the movie? (or did you just know about the story from your life curiosity?)
Never saw the movie. I was in Wales to research my family history. Mered and Phyllis spent lots of time doing research on old Welsh music. Some of their "finds" were passed along to Linda Griffiths whose group Plethyn has performed and recorded the music for posterity. Mered and Phyllis suprised me with the original document from the National Library of Wales in Aberyswyth that recorded the adoption of my great grandfather by his uncle in Llanfihangel-y-Creuddyn. I was beyond surprised. My great grandfather went on to emigrate to the US and fought in the Union Army in 1863.
The crossover was pure coincidence between my travels for family history and the current political changes in Wales. I'm still studying Welsh to facilitate reading old records that remain in Welsh. Very handy when walking through a cemetery to read the headstones or reading old parish records.
An example of the kind of music Mered and Phyllis found in the library and passed to Plethyn is Tan yn Llyn. Lyrics in Welsh and English in the music video.