Do not pay to visit this site!
As already mentioned, entry to the grounds of Plas Newydd, does not allow access to the site. Park up on the visitors car park, ignore the ticket office, and walk down to the picnic bench beside the fence. The site is visible over the fence. To gain access, I presume it’s a case of ringing/writing for permission OR going on one of the guided archaeological tours. It’s £3.00 and takes two hours, so some of the walk probably takes in the gardens, which may or may not be of much interest!