The Gates of Glory have actually both falllen, and are close to the Milestone.
I was down there not too long ago and parked by the Milestone. I'm a bit miffed I missed RA near to it ... I shall have to go back :-)
The Dingle Inventory mentions only one piece of RA in Milltown and it is 20m from The Milestone. It seems that the one marked on the OS map is incorrect and is the stone you found. That stone should have ... a single cup at one end of the east face and ... wait for it ... 18 isolated cup marks and 8 cup and circle motifs, most of which have radial lines. Beneath these is a "complex grouping of cup-marks and straight and curved lines". There are three further cup-marks on the WSW face. It was Ó Nualláin that suggested the two stones here once stood and that the decoration was added later (because the grooves follow the downward slope.)
I saw this stone over the fence, but didn't go into the field to look at it, because I thought the RA was over the road. I'm now more than a little miffed.