Talking Heads in Irish Mythology

A severed head depicted in Celtic artwork does not a macabre ritual make. However, it’s fair to say that the severed head makes many appearances in the ancient stories of Ireland. But scholars are now revising their opinions; it seems the Celts were removing the heads not of their enemies, but of their beloved friends who had fallen in battle. Could the same thing have happened here in Ireland? And if so, why would they behead their friends?