Saint Patrick himself was kidnapped from his home in Britannia and sold into this practice. He would return to Ireland as a man to spread Christianity. This practice was effectively abolished in the Isle with Norman conquests of Ireland.
Slavery
Ireland is the referred to as "The _______ Isle" after this precious green gemstone. The pokemon game also with this name features Rayquaza on the front!
Emerald
Again in the 12th century, the Norman conquerors of England turned their attention to Ireland. The Norman's ended this marital practice that now only 2% of the world's households have.
Polygyny
James Joyce wrote this seminal novel, it shares a name with a former United States President and somewhat shares the name with the central character in Homer's Odyssey.
Ulysses
Julius Caesar recorded the Gauls in France as this term, which referred to a larger ethnic group. Caesar fought a coalition of these people led by Vercingetorix, son of Celtillus, this started the migration pattern of these folks north, who would later settled Ireland.
Celts!