Bromwich's edition of the Welsh Triads is considered the definitive reference.  It is the standard English Translation of the 126 Triads by Iolo Morganwg which is found in Iolo Aneirin Williams MS 71 in the National Library of Wales. A new version has been rumored for years and is finally being released (see Below).

Bromwich R: "Trioedd Ynys Prydain: The Myvyrian 'Third Series" Transactions of the Honourable Society of Cymmrodion 1968 Part II pp 299-338 for Iolo Williams' Triads 1-50 plus Notes, and 1969 Part I pp 127-155 for Triads 50-126 plus Notes.

Trioedd Ynys Prydein
The Triads of the Island of Britain 3rd Edition
Edited with introduction, translation and commentary by Rachel Bromwich
pp cxvi559 234x156mm hardback February 2006 65.00
ISBN-10 0-7083-1386-8
ISBN-13 978-0-7083-1386-2

Gordd Cymru Welsh Triads Collection