I decided to ask at a house in the area and as luck would have it I found the land-owner who was kind enough to allow me access to where the stone was thought to be. He had heard the story linked to the area but didn't know what specific stone it referred to.
Dermot Gleeson wrote about the track in the North Munster Antiquarian Journal in 1958. On the 1840s Os map on this track is marked "Lurganboe".
He says
"The only other tradition of the saint I know of in the Ormond area concerns not himself but his cow. This is the "Rinne Bó Phadruig" or track of St. Patrick;s cow at Grennanstown in Toomevara. On the ancient road, part of which still remains, between Latteragh and Tyone and just after it passes by the road from Ballinamona cross to Grawn, is found a large stone by the roadside with a depression in it said to have been made by the knee of the saint's cow when she fell while running from the devil"
There are two candidates for the stone one in situ where it seems to be indicated on the map and one at the man I talked to house. He told me that he brought a few stones from the field to his garden many years ago.
With the first one (above and directly below), a depression seemed to be filled up with clay over the years.
The second one below looks a lot more lie a knee depression that could have been associated with the legend.
I think myself the first is the most likely of the two.
The Lurganboe isn't a recorded monument and is likely to have been a natural rock or stone that became associated with a legend and so there is no issue with moving it (if this has occurred).
It is nice to be able to record these stories. Personally I feel they make the landscape come alive and give people more of a connection with it.
Thanks so much to the local man to allowed access to his land and for all his help and information.