Looks like Ffynnon Eidda to me.
Where the minor road south from Carrog meets the B4407 stands Ffynnon Eidda in lonely grassland.
It bears a plaque proclaiming it’s restoration in 1846. Eidda was in all probability a saint but there is no certainty of this.
The book “Ffynhonnau Cymru” states that the well was a drover’s well used by their animals as they travelled across the Migneint.
The Welsh inscription above the well reads "Yf a bu Ddiolchgar" (Drink and be Grateful'). However, if you did indeed drink from it you would probably contract dysentery.