Music and arrangement by Joseph Parry, lyrics by Richard Davies (Mynyddog). First published 1875.
Richard Davies’ lyric may have been influenced by the 14th Century love story of the poet Hywel ab Einion Llygliw and the princess Myfanwy ferch Iorwerth Ddu of Castell Dinas Brân, Llangollen. That story was the subject of the Eisteddfod prize winning poem, Myfanwy Fychan o Gastell Dinas Brân (1858), by John Ceiriog Hughes (1832–87). The popularity of this poem revived the Myfanwy tradition. Alternatively some sources say Myfanwy was written with Parry’s childhood sweetheart, Myfanwy Llywellyn, in mind.
Myfanwy Fychan describes the (unrequited) love of the poet Hywel for Myfanwy. Myfanwy was born in the mid 14th century and her family lived at Castell Dinas Brân, a hilltop castle near Llangollen whose ruins can still be seen from far away. Her father being the Earl of Arundel owned the land thereabouts.
Unsatisfied with various suitors she allows poet and harpist Hywel ab Einion Llygliw to visit her and he transfixes her by playing and singing of her beauty. He falls in love with her and thinks she feels the same way, but as much as she loves his poetry she does not return his love. Eventually she married Goronwy Fychan ap Tudur and apparently he also wrote many poems to her, as did other poets too. Maybe she just really loved poetry.
Amazingly, Hywel ab Einion’s poem survives although I wasn’t able to find the Welsh version! You can see the (fairly illegible) manuscript and translation here: http://www.llangollenmuseum.org.uk/MythsAndLegends/DinasBran/Myfanwy.htm.
Some analysts think that ab Einion’s poem was written after Myfanwy was married (as the title uses her married name Myfanwy Fychan), possibly after the amour courtois tradition of singing to married women, in which case the story of his unrequited love for her could be a later invention.
Sources
https://en.wikipedia.org/wiki/Myfanwy
https://coflein.gov.uk/en/site/307064/
https://www.bbc.co.uk/wales/history/sites/themes/society/myths_myfanwy.shtml
https://en.wikipedia.org/wiki/Myfanwy_Fychan
Words
Paham mae dicter, O Myfanwy
Yn llenwi’th lygaid duon di
A’th ruddiau tirion, O Myfanwy
Heb wrido wrth fy ngweled i?
Pa le mae’r wên oedd ar dy wefus,
Fu’n cynnau ‘nghariad ffyddlon ffôl?
Pa le mae sain dy eiriau melys,
Fu’n denu ‘nghalon ar dy ôl?
Pa beth a wneuthum, O Myfanwy,
I haeddu gwg dy ddwyrudd hardd?
Ai chwarae’r oeddit, O Myfanwy,
 thannau euraidd serch dy fardd?
Wyt eiddo im drwy gywir amod,
Ai gormod cadw’th air i mi?
Ni cheisiaf fyth mo’th law, Myfanwy,
Heb gael dy galon gyda hi.
Myfanwy, boed yr holl o’th fywyd
Dan heulwen ddisglair canol dydd,
A boed i rosyn gwridog iechyd,
I ddawnsio ganmlwydd ar dy rudd;
Anghofia’r oll o’th addewidion
A wnest i rywun, eneth ddel,
A dyro’th law, Myfanwy dirion
I ddim ond dweud y gair “Ffarwel”.
Translation
Why is anger, O Myfanwy,
Filling your black eyes?
And your gentle cheeks, O Myfanwy,
Not blushing on seeing me?
Where is the smile that was on your lip
Which ignited my faithful fond love?
Where is the sound of your sweet words,
Which attracted my heart after you?
What thing did I do, O Myfanwy,
To deserve the scowl of your beautiful cheeks?
Were you playing, O Myfanwy,
With the golden flames of your poet’s love?
You belong to me through a true promise,
Is it too much to keep your word to me?
I will never try your hand, Myfanwy,
Without having your heart with it.
Myfanwy, may the whole of your life
Be under the shining midday sun.
And may the blushing rose of youth
Dance for a hundred years on your cheek.
Forget all of your promises
That you made to me, pretty maiden,
And give your hand, gentle Myfanwy,
Only to say the word “Farewell”.