The unlikely pairing have been chosen as the Friday and Saturday headliners for the comedy stage at the event, which this year takes place at Old Warden Park over the bank holiday weekend of August 24-26.
Ardal is best known for his role as Father Dougal in the classic 90s comedy Father Ted.
But there is far more to his repertoire than simply playing a simple-minded holy man. Ardal has played to sell-out audiences across the world with his stand up routines, has produced two top ten comedy DVDs, and his debut book Wide Open Spaces is a best-seller.
Meanwhile, John Cooper Clark - it seems wrong to call him by anything less than his full name - has been recognisable in his slum- hipster-chic style for more than three decades. His polysyllabic performances have to be seen to be believed.
The rest of the all-star comedy bill will be announced shortly.
Music acts appearing will include Hawkwind, Geno Washington, Booker T, The Damned and the Boomtown Rats.
Weekend tickets for the 2012 Rhythm Festival cost £99.
For more information visit www.rhythmfestival.com