Tara has been dug into twice by people who were certain they knew what was there, and neither was an archaeologist.
The Ark. Between 1899 and 1902 a group of British Israelites, who held that the Irish were descended from a lost tribe of Israel, excavated at Tara looking for the Ark of the Covenant, which they believed had been carried to Ireland. They dug into the Mound of the Hostages area and the surrounding earthworks and damaged the monument. They found nothing, because there was nothing there.
What makes the episode significant is the response. The dig was publicly opposed by Douglas Hyde, Maud Gonne, Arthur Griffith and W. B. Yeats among others, who visited the site in protest and campaigned in the press. It became one of the first organised heritage campaigns in Ireland, and it fused cultural nationalism with monument protection, the argument being that Tara belonged to the Irish nation and that a British sect digging it up for a biblical relic was of a piece with everything else.
The motorway. The second fight ran from 2007 to 2010, when the M3 was routed through the Gabhra valley below the hill over sustained protest, direct action and litigation. During construction a previously unknown henge at Lismullin was discovered, a substantial Iron Age ceremonial enclosure that nobody had known existed, and whose presence was itself an argument about how much of the Tara landscape remained unrecorded. It was excavated and removed, and the road was built.
What is actually there. Tara is a low ridge with a very long view, carrying earthworks of several periods: the Rath na Riogh enclosure, the Forradh, and on the summit the Lia Fail, the stone of destiny, said to cry out under the rightful king. The inauguration of the kings of Tara carried a claim to the high kingship of Ireland, and the title retained enormous symbolic weight long after it meant anything administratively.
The oldest thing on the hill is much older than any of that. The Mound of the Hostages is a small passage tomb of around 3200 BC, and its entrance is aligned so that it takes the sun at the beginning of November and the beginning of February, the points that later became Samhain and Imbolc. Whoever built it was marking the same divisions of the year that Irish tradition kept marking for the next five thousand years.