Earliest surviving manuscripts date from the (12th, but preserve an earlier oral tradition.
Cyfraith Hywel (or Laws of Hywel Dda) - an early (10th compilation, showing a distinctive tradition from that of the Anglo-Saxon.
Note. If your interest is in medieval Welsh law - be sure to visit Gwefan Cyfraith Hywel Website
There is also an unpublished Oxford DPhil thesis (1952) The Book of Iorwerth (Llyfr Iorwerth) : a critical and definitive text of the Gwynedd (Venedotian) Code of medieval Welsh law by by Aled I.R. Wiliam (MS. D.Phil. d.1159) which can be consulted in the Old Bod.
Statute of Rhuddlan (aka Statute of Wales) 1284, 12 Edw 1 - English royal following execution of Dafydd ap Gruffydd & annexation of his principality in 1283. To introduce English criminal law, but retain Welsh custom and law in civil proceedings. Government rearranged with the formation of 6 sheriffdoms:Anglesey, Caernarfon, Merioneth, Flint, Carmarthen, and Cardiganshire.
Statute 28 Edw III c.2 1354 Welsh Marches brought under English crown but allowed to keep own customs
Statute 27 Hen VIII c.26 1536 Laws and liberties of England extended to Welsh subjects, but local custom could still be proved.
Stat 34 & 35 Hen VIII c.26 1543 confirmation of 1541 ordinances setting up Great Sessions in Wales
1830 England & Wales become a unified jurisdiction
Since its foundation in 2000, the Welsh Legal History Society has published a series of works (scholarly editions, collections of essays, and monographs across a variety of periods and subjects): it is therefore worthwhile doing an exact phrase search in SOLO based on Welsh Legal History Society.
Cambrian Law Review Cw UK 300 C15 For holders of an Oxford SSO, HeinOnline has Vols. 1-46 (1970-2016)
This journal is a general law journal, but is also another source to turn to if looking for aspects of Welsh legal history.