I’ve been using the Ap Geiriaduron on my phone for a while now, but as my level of Welsh has got more literary I’ve been finding that there are more and more words that I can’t translate on there. So I’ve been using the online version of the Geiriadur Prifysgol Cymru instead.
But I found out today that the GPC is launching an ap:
http://www.bbc.co.uk/cymrufyw/35644789 (sorry - story in Welsh only).
I’ve just downloaded it, so it definitely works on an iPhone 