One of a pair, the whereabouts of the second head is unknown but it is one of the latest antiquities stolen for sale to collectors in Britain, Germany and the United States according to police and museum curators.
The life-sized grey stone head-and-shoulders effigies of a bishop looking down on an abbott date from the 1420s. They disappeared separately from the remains of an early Christian church in Kilmacreehy in August 1992 and October 1993.
One has been found by a local farmer hidden on his land. Police believe they know the identity of the thief, and hope to recover the second head.
Other thefts in Co Clare have included conical stones from toll gates at Sixmilebridge and items from graveyards.
Garda John Faul of Lahinch, who has worked in the county for 25 years, said: "There are stones going all the time from sites all around the Burren. It's a pity because none of these things can be left out any more. For a while everything was left out and nothing would go. There were also a lot of antiquities on farms which people didn't realise were important."
Experts say London is the single biggest market for stolen or illegally exported Irish relics. "The British are notoriously bad at introducing either internal controls to protect heritage or co-operating internationally," one Irish expert complained.
Recently, controversy surrounded the export to Britain of two bronze cannon made by the Owen brothers, gun-founders to King Henry VIII and Edward VI. They were removed illegally from a wreck in Tramore Bay, Co Waterford. Their legal ownership is now being contested between the Royal Armouries in London and Ireland's National Museum.