I'll just quote the book "Celtic Baby Names" by JudySierra, but you'd better wait until Merriment confirms (as there have been a couple of mistakes in that book already!):
Iolo m. (YOH-loh) A common nickname for Iorwerth, often used as a name in its own right. Iolo Morganwg (Iolo of Glamorgan) was the bardic name of Edward Williams (1747-1826), stonemason, poet, scholar and initiator of the modern National Eisteddfod.
Iowerth m. (YOHR-wayrth) From Norse 'iôr' "lord" + Welsh 'gwerth' "value, worth". From the Middle Ages to the present, Iorwerth has been use as a Welsh version of Edward.