I found this translation online at http://www.celticartscenter.com/Songs/Welsh/Myfanwy.html. It follows the original Welsh quite closely.

Why is it anger, O Myfanwy,
That fills your eyes so dark and clear?
Your gentle cheeks, O sweet Myfanwy,
Why blush they not when I draw near?
Where is the smile that once most tender
Kindled my love so fond, so true?
Where is the sound of your sweet words,
That drew my heart to follow you?

2. What have I done, O my Myfanwy,
To earn your frown? What is my blame?
Was it just play, my sweet Myfanwy,
To set your poet's love aflame?
You truly once to me were promised,
Is it too much to keep your part?
I wish no more your hand, Myfanwy,
If I no longer have your heart.

3. Myfanwy, may you spend your lifetime
Beneath the midday sunshine's glow,
And on your cheeks O may the roses
Dance for a hundred years or so.
Forget now all the words of promise
You made to one who loved you well,
Give me your hand, my sweet Myfanwy,
But one last time, to say "farewell".