Tara (Ir. Teamhair) is chief among the best known ‘royal’ sites of Ireland, including Dún Ailinne in Leinster, Crúachain in Connacht and Eamhain Mhacha in Ulster. It is at once a landmark and a vantage point. The barrows, mounds, conjoined circular earthworks and buildings, timber henges, linear embankments and sacred springs and marshes at these royal centres together make up prehistoric ‘ritual landscapes’ that have evolved over many centuries. This was the centre of an exceptional sacral kingship and the title rí Temro 'king of Tara' was fiercely contested to the 8th century. The names of the monuments at Tara, known from 11th-century topographical texts, are steeped in mythology and saga literature.
An examination of the surviving monuments tells us that the Hill of Tara was primarily a ceremonial site, a place where people came to bury their dead and where important religious and royal events were celebrated by prehistoric communities. During the later prehistoric period it may have been guarded by the nearby fortified sites of Rath Lugh, Ringlestown Rath and Rath Miles. It may not have been until relatively late in its development that defence became a consideration at Tara itself. Ráith na Rí ('Fort of the Kings'), one of the major enclosures at Tara, like those at Dún Ailinne and Eamhain Mhacha, has its bank placed outside the ditch (or fosse). This appears to be a non-defensive arrangement which is likely to have marked the inner sanctuary on the hill. The name Teamhair itself actually means 'a place set apart, a sanctuary'. Rather than deny access from without, the intention was to contain a potent, otherworldly force. It is possible that the practice of using internally-ditched enclosures to define sacred space belongs to a long standing tradition from the Late Neolithic to the Iron Age. It seems unlikely that there was ever a permanent resident population at Tara in the prehistoric period except perhaps for religious guardians of the sanctuary.
There are over thirty visible monuments on the Hill of Tara and as many more that have no surface remains but which have been identified using geophysical prospection techniques and aerial photography. Most of the monuments on Tara can only be dated by comparison with others of known date elsewhere. Three monuments have been excavated, the Mound of the Hostages, one of the earliest monuments on Tara, the Rath of the Synods and part of Ráith na Ríg. Results of these excavations give a general time bracket for the monuments on Tara extending from the Neolithic (c. 3500 BC) to the later Iron Age (c. 400 AD). It is sometimes possible to establish a relative chronology for the monuments. For instance, the ramparts of Ráith na Rí were laid out in such a way as to avoid the Mound of the Hostages, demonstrating that Ráith na Ríg is a later monument.
The earliest monument so far identified is a large, probably palisaded enclosure around the summit of the hill that was built during the Neolithic period. Part of this enclosure was found beneath the Mound of the Hostages. Only a few enclosures of this date are known from Ireland, e.g. at Knowth, County Meath and Lyles Hill, County Antrim, and although domestic activity is attested in some of them, in the majority of cases there is also evidence of ritual activity associated with death. Evidence from Britain suggests that comparable Neolithic hilltop enclosures may have been used specifically for seasonal gatherings.
Most of the monuments at Tara are barrows - circular burial or funerary monuments consisting of a low earthen mound surrounded by a ditch and sometimes an outer bank. From the one hundred or so of these that have been excavated in Ireland it is evident that they remained in use from the Late Neolithic to the first few centuries AD. Although barrows cannot always be dated on the basis of outward appearances alone, it is likely that the majority of those at Tara date to the Bronze Age. Time and erosion, agriculture and soil regeneration have reduced many of them to barely perceptible circles in the grass. There is a particular concentration of barrows along the north-western and northern flanks of the hill, the most significant of which are the Cláenfhearta ('Sloping Trenches')..