The earthwork on the hill is a quadrivallate (four banks) ring fort with a diameter of 140m, and ditches between them.
Damage to the earthwork is blamed on Oliver Cromwell's army which camped on the site in 1649.
Tlachtga was a powerful druidess in Irish mythology.
While in Italy with her her Druid father Mogh Ruith where they were studying with the sorcerer Simon Magus,
Tlachtga was raped by Simon's three sons.
Tlachtga returned to Ireland and gave birth to three sons; Dorb, Cuma and Muacth and then died.
She was buried on the hill which now bears her name.
In Celtic times the top of the hill was sacred ground and only open to the druids except on the Festival of Samhain
when members of the public were permitted.
Samhain celebrated on the night of 31st of October every year (precursor of Halloween).
Samhain marks the end of the old Celtic Year and the beginning of the New Year.
Local folklore says the druids practiced human sacrifice and that Tlachtga was a place
of pilgrimage for women who were childless. The women would bring their slave's children
to be sacrificed so that the spirit of the child would enter their body and be reborn as their child.
After sunset the ceremonial New Year Samhain fire was lit on Tlachtga (Hill of Ward). Torches were lit from
this sacred fire and carried to seven other hills around the county including Tara.