The Geiriadur Prifysgol Cymraeg (GPC) often has etymologies of words - it is available on-line at http://geiriadur.ac.uk/gpc/gpc.html and as a smart-phone app. You can choose either Welsh or English as the interface language, and if you hover on abbreviations their meaning will pop up.
(Be careful if using it as a general dictionary - as it gives historical meanings, sometimes back as far back as Early or Middle Welsh, as well as recent ones, you can end up trying to use a word that no longer has a particular meaning in the contemporary language.)
the fact that it has very old and archaic meanings makes it even cooler. I always enjoy finding proto indo-european words in modern PIE languages and I'm guessing the further back you go with Irish and Welsh literature the more closer them languages get? Thanks anyhow I'll download that now