This shows a "Capel" on the site of "Saron Chapel" which is marked on later maps.
https://maps.nls.uk/view/102182700#zoom=6.7&lat=3054&lon=6548&layers=BT
Mabel,
Could you pl check this item from Welsh Journals - ""Tywysydd y plant 1890.".
https://journals.library.wales/search?rows=10&sort=score&order=desc&alt=&query=%22old%20harp%22&range[min]=1735&range[max]=2007&decade[]=1890&page=1
Would you take it to mean that Saron is sited on the Old Harp or that it simply kicked off there?
Added - seems there's a problem with URLs.
Mabel,
You can find the article on Welsh Journals searching for "Old Harp"
The item from Tywysydd y Plant also relates the life history of Rees Morgan. The end of the item on the page linked to says:
According the the History of Independent Churches, Mr Morgan's parents were a great support to establishing the cause in Troedyrhiw, for it was in their house - the Old Harp between Troedyrhiw and Dyffryn - that Saron started. Mr Methusalem Jones, Bethesda, Merthyr and the supporting preachers connected to Bethesda, would preach in the Old harp every Sabbath evening for several years, and the Sunday School was set up in the school house where his father ran the daily school. His father and mother were among the 29 members that established Saron. Mr Morgan remembers being a very small child with his father in the midst of thorns and growth measuring the land where the chapel and cemetery now stand in order to prepare the request to the Dowlais Company for the land to build a place of worship
So I think it's clear the group that set up Saron started holding their meetings in the Old Harp before they got the land where Saron stood (and the cemetery remains)