Ieuan played with Gruff Rhys in their early band Ffa Coffi Pawb. He was a drummer for Welsh band Catatonia from 1993-1996, recording on their early singles and debut album Way Beyond Blue released in 1996. During the same period Super Furry Animals formed and Ieuan left Catatonia to focus on them.
Ieuan runs the Cardiff based label and recording studio Strangetown Records with brother Cian Ciaran and Mick Hilton. With Ciaran, he was awarded by BAFTA Wales 2011 for Original Music for the S4C drama Pen Talar filmed by Fiction Factory.
Ieuan released The Golden Mile with The Peth ("The Thing" in Welsh) in 2008, with Welsh actor Rhys Ifans on vocals. A second album, Crystal Peth has been recorded but has yet to be released. With DJ and rapper Rashid "Wibidi" Omar, he recorded and produced music under the band name Wibidi. In 2013, he recorded with The Earth (band), with guitarist Mark Roberts (formerly of Catatonia), bassist Tristan Marley and vocalist Dionne Bennett.
He is musician Cian Ciaran's older brother and he has two sisters. His father, Dr. Carl Iwan Clowes OBE, is health consultant, Welsh language campaigner, anti-nuclear campaigner and Honorary Consul in Wales for African country Lesotho.