This page in ‘Traces of the Elder Faiths of Ireland’ by W G Wood-Martin (1902) seems to show the same cross and base as Tiompan’s picture from Boho churchyard (about 500yds from the rock carvings). The book says: Men are reported to resort to this stone in cases where they have no children after marriage.
I don't think it is the same Rhiannon .The one from Welch doesn't look to be in a churchyard or at least not Boho churchyard , and the bulluans are beside it rather than on the cross slab .
It says on p247, 'The cross at Boho near Enniskillen, has a bullaun at either side of the base... (fig 73)' and the photo I've linked to is fig 73? Does it not match your photo? I just thought the word 'bullaun' was a bit hopeful, that maybe they meant the little dips. Are you saying Wood-Martin got the wrong photo, it wasn't what Welch meant?
Just had a search and it looks like the cross has been moved to the churchyard in the recent past so it is probabaly the same cross but in a different location from the Welch pic .
ah so the bullauns are Lost?
It gets even more complicated .Maybe the bullauns are still at the origianl site or one possibly the under the cross slab ??
qub.ac.uk/schools/CentreforArchaeologicalFieldworkCAF/Reports/DataStructureReports/Filetoupload,64239,en.pdf