One of the great assemblies or fairs [OIr. óenach; ModIr. aonach] of early Ireland, and the druid's daughter for which it was named. Tlachtga, a sorceress, was the daughter of Mug Ruith, the archdruid of Ireland, from whom she learned much secret wisdom; she travelled with him to the world's seats of learning and brought home magical stones from Italy. After having been raped by the three sons of Simon Magus (often associated with her father), she gave birth to triplets (see TRIPLISM) on the hill in Co. Meath that was to bear her name. Like Macha (3), she died of grief after the children were delivered; over her a fortress was built.
The fair or assembly of Tlachtga took place on a site now identified as Ward Hill (390 feet), 2 miles E of Athboy, 12 miles W of Tara, 8 miles SW of Tailtiu, Co. Meath, whose celebrations were comparable. Like Uisnech, the Ward Hill site contains the ruins (much disturbed in Cromwellian times) of a massive ring-fort, consisting of four concentric banks and ditches, surrounding a 25-foot platform crowning the hilltop. The focus of the ceremonies, thought to have been begun by Lug Lámfhota, was the lighting of the winter fires at Samain [1 November]. Although the rulers of the intermittent kingdom of Mide sat here, Tlachtga was often associated with the province of Munster; the site lies in the contemporary county of Meath, a part of modern Leinster, in territory coextensive with the medieval kingdom of Brega. The fabulous flying-machine constructed by Mug Ruith and Simon Magus, roth rámach [Ir., rowing wheel], was sighted here. At Tlachtga in 1168 Ruaidrí Ua Conchobair, last ard rí [high king] of Ireland, presided over a national synod of kings and prelates; 13,000 horsemen are said to have attended.