As far as I know, Over The Top Tours are only providing a shutlle service from Dublin to Newgrange.They cannot guarantee you entrance to Newgrange/Knowth. From their leaflet that I picked up recently, they depart twice a day from Dublin - 8.45AM and 11.15AM and return at 1.30PM and 4.30PM. The journey will take about an hour.
The bus brings you to the Brú na Bóinne Visitor Centre, where you then buy your entrance ticket, and the the visitor centre then provides its own shutlle bus service to bring you to Newgrange and/or Knowth, where they provide a guided tour. You have the option of visiting just one or the other or both sites. There is also a very good interpretive centre in the visitors' centre.
I visited there, just last week, and opted to visit both sites. You will need at least three and a half hours, if you want to visit both sites, and then give yourself a further hour for the interpretive centre - and a cup of coffee!.
I think it is worthwhile visiting both sites. But, if I had to choose only one, then I would opt for Newgrange, mainly because you can actually enter the inner chamber and when there, they re-enact the effect of the sun rising in the winter soltice, filling the chamber with its light. By the way, I had my own transport, so I did not have any time constraints
And so, if using Over The Top Tours, or any other service for that matter, I would travel on their earliest departure, in their case 8.45AM, and return on their 4.30PM departure. I think if you intend to return on their 1.30Pm bus, you are leaving yourself too tight on time, especially if you want to see both Knowth and Newgrange and also the interpretive centre. You also have to allow for the fact that you may have to wait at the visitors centre to get a tour time. We arrived last week at 11.45AM, were lucky to get a Knowth tour for 12.15PM and had to wait until 2.15PM for a Newgrange slot. So, the earlier in the day that you can get there, the better.I imagine Sepember will not be quite as busy as it was last week, and if you are up there by 10 -10.30 AM, you should have no difficulty getting in. Over The Top Tours phone number, if you need it, is: ++353 1 8386128