Yet another translation, courtesy of Singing Together (Summer 1954) as I'm working on that thread at the moment:

SUO GAN
Welsh Lullaby
Translated by M Ll Davies

Sleep my baby, rest my loved one,
Softly slumber now with me.
Clasped in mother's arms so tender,
Warm in mother's love for thee.
Naught shall ever come to harm thee
While my loving watch I keep.
Thou my pretty one shall slumber
While I sing thy lullaby.

Sleep my baby, rest my loved one,
While the evening shadows creep.
Why, my birdie, art thou smiling,
Smiling sweetly in thy sleep?
Can it be that all the angels
In God's heaven smile on thee?
Rest my darling, smile and slumber
While I sing thy lullaby.