I stumbled upon this while watching a history documentary
https://en.wikipedia.org/wiki/Y_Beibl_cyssegr-lan
It mentions the use of Hebrew and Greek source text for the translation.
However it doesn't specify which group of Hebrew or Greek texts.
eg.
Which Hebrew - Massoretic or Septuigent?
Which Greek - TR or Critical text?
1 Answer
An Act of Parliament of 1563 required the Welsh bishops and the Bishop of Hereford to produce a Welsh translation of the Bible and the Book of Common Prayer, and stipulated such should be used in areas where Welsh was the commonly spoken language (which then included parts of Herefordshire in England).
Although the New Testament and Prayer Book appeared in 1567, the full translation was under5taken by William Morgan, encouraged by Archbishop Whitgift of Canterbury. Morgan later became Bishop of Llandaff and then St Asaph's.
All the translations into English at that time used what became known as the Textus Receptus for the New Testament and the Masoretic Text for the Old Testament. These included the Geneva Bible, the Bishops' Bible and the King James Bible. As the 1588 Welsh Bible was very much "official" and approved it must have used the same.
In any case the Critical Text was not published until some centuries later and the statement that the Old Testament was translated from the Hebrew rules out the Septuagint because it was Greek.
The Bishops Bible, translated to English in the 1560s, used the Stephanus Greek version of what came to be called Textus Receptus, and the Munster 1535 version of the Hebrew Masoretic Text. The King James was based also on this and it seems extremely likely the Welsh Bible was also.