Indeed it is and that still happens. This was one of the Lugnassa festivals. (Alledgedly) St Patrick visited during one of these festivals and spent the obligatory 40 days & 40 nights on the hilltop banishing serpents. As snakes never existed in Ireland it can only be assume that he (or one of the other monks that modged together and became Patrick) put an end to pagan practices on the hill.
The festival was originally only for women and childless women would sleep up there (presumably with Lugh who lived in the cairn.)