Karl spent many hours writing radio comedy in the late 20th and early 21st century. Not many people post-1960 proclaim their life's ambition 'to write for the radio' but Karl found himself strangely attracted to the medium. Why was this? Did he see a chance to explore a cornucopia of sounds and images in this truly intimate medium? No. Not really. They accepted his scripts.

BBC Radio Four produced his six-part comedy series The Mahaffys 'Ballykissangel meets The Simpsons' in 1997 starring Pauline McLynn. It was described as a 'surreal Irish Archers' by The Daily Telegraph and The Mail On Sunday said it was a 'very funny comedy with daft Irish humour'.

In 2001 Karl wrote and appeared in a half-hour comedy on RTE Radio One called A Smaller Than Life Character.

In 2002 RTE Radio One aired the six-part spoof documentary series Gone But Forgotten. Telling the story of six justly neglected minor historical (and at times hysterical) figures from various strands of Irish culture, it starred Karl himself along with Morgan Jones, Karen Ardiff, Deirdre Monaghan and Conor Lambert.

In 2003 Karl wrote a specially commissioned RTE half-hour radio comedy Give Pearse a Chance commemorating the 87th anniversary of the 1916 Easter Rising. It went down a treat in West Belfast.

Karl's two season RTE radio sit-com Here's Johnny about a New Jersey mobster and his wife relocating on a witness protection scheme to leafy south Dublin appeared in 2006. Morgan Jones, Tara Flynn and Michael McElhatton starred. It was described as 'tight script, first-class acting, pacy, funny' by The Irish Times.

For a man who referred to laughs as Godot during his stand-up comedy days - sometimes he spent a long time waiting for one - Karl also wrote a radio drama homage to Samuel Beckett, Conversations in The Dark, which was broadcast as Play Of The Week on RTE Radio One in September 2004. That great theatrical exponent of Beckett, Barry McGovern, took on the leading role of a recently deceased corpse who has a lot on his mind.