Tlachtga is one of very few quadrivallate ringforts in Ireland and a site that has long been regarded as of potential 'Royal' status. It is a site with significant associated folklore and historical narrative associated with it. In the 12th century, Tlachtga was the scene of negotiations between Hugh de Lacy and Tighernan Ui Ruairc, king of Breiffne, at which Ui Ruairc was said to have been slain by treachery and his head sent to Henry II in London.

Initial remote sensing survey in 2011-12 using both lidar and multi-method geophysical survey highlighted the presence of multiple, partially overlapping phases of enclosure at the site. Three subsequent seasons of excavation yielded over 15,000 fragments of animal bone, human remains, charred material, evidence of metalworking, and a hoard of 23 Anglo-Saxon silver pennies. The earliest material culture comprises lithics and two small sherds of middle Neolithic pottery.

The main activity at the site seems to span two periods: a late Bronze Age to early Iron Age (1100-400 BC) trivallate hilltop enclosure and a late Iron Age to early medieval (AD 400-1000) enclosure phase. This latter in particular sees a great deal of development from a 40 m univallate enclosure to a 150 m quadrivallate enclosure, and finally a phase of mound building where the central space is converted to a 'rath mound' probably in the mid-10th century. This is contemporary with the deposition of the coin hoard just east of the main complex in an apparent craft-working area. The final phase examined seems to indicate a timber stockade constructed on the central mound, most likely in the 12th century.