(Here's a bit about the most famous Irish witch, Dame Alice Kyteler, adapted from a Moon Ireland sidebar.)
Easily the most colorful character in Kilkenny history, Dame Alice Kyteler was a businesswoman in the early 14th century who gained more money and power with each husband she acquired. There were four in all, and since each died under mysterious circumstances it was inevitable that Dame Alice should be accused of witchcraft. According to Peter Somerville-Large in Irish Eccentrics, Dame Alice allegedly led her coven in parodies of the Mass using dead men's fingernails and the shrouds of unbaptized boys. "She sacrificed nine red cocks and nine peacock's eyes to her incubus, Art, or Robert, who had carnal knowledge of her in the shape of a cat [or] a hairy black dog..."
Though formally charged in 1324, her influential friends (her brothers-in-law, mostly) had the offending bishop, Richard de Ledrede, imprisoned for seventeen days. The trial commenced upon his release, however, and Dame Alice and her servant girl, Petronilla, were sentenced to burn at the stake. Dame Alice fled the country the night before the execution, leaving loyal Petronilla to her fiery fate on the third of November, 1324. (Somerville-Large gives the 3rd of September, 1325 as the date of execution.)
Alice Kyteler's firstborn son, William Outlawe, agreed to give alms to the poor and re-roof the choir stalls at St. Canice's Cathedral to avoid the gallows. Dame Alice was never seen or heard of again.
If this story captures your imagination as it did mine, you'll want to hunt down a copy of Emma Donoghue's The Woman Who Gave Birth to Rabbits (Half.com link here). "Looking for Petronilla" is the best of the collection.