At the outset there seems no reason why the Ffynnon Fair (St Mary’s Well) at Penycae, near Wrexham, should be where it is – quite distant from any settlement. Indeed, there isn’t total agreement as to where it is since the Historic Environment Record (HER) places it a few hundred yards to the north near Trefechan. However, the 1840 tithe maps record a field Cae Ffynnon Fair (pictured below) which surely must mark its true position, just to the south of Trefechan Farm and a little to the north of the ruins of Plas Du (header picture).
William Schleising wrote in 2009, …another Mary’s well, Ffynnon Mair. About half way down the lane in Drefechan, Penycae, between Drefechan Farm and the ruins of Plas Du (Plas Ty). The well used to be accessible to all from the lane, it was our only source of water when we lived in Plas Du in the late 1940s. This is no longer the case as it is now fenced off from both the lane and the general public. Access now is only possible from the adjacent field, Cae Ffynnon Mair. Holy or not, it was our spring of life. 
The spring certainly has a long history , it was known before 1620 when it is mentioned in a survey of the lordship of Bromfield. The site was visited in 1911 by the Royal Commission for Ancient and Historic Monuments in Wales, but they did not record any specific description of the site -implying that there was little to be seen, and note only that there are no particular traditions associated with the spring.
The village of Penycae itself only developed significantly in the mid 19th century, prior to that it was a rural area contained within the parish of Ruabon. However, local tradition suggest that the area does have links with early Christianity, the legend has it that it was here that monks fleeing from the destruction of the monastery at Bangor Is y Coed following the Battle of Chester in 616 were rounded up by the Anglo Saxons here and put to death by hanging on a yew tree at the site bearing the name Y Groes. The current village of Penycae has grown around the historic settlements of Pentre Cristionydd (Christian village) and Dynhinelle.
It would appear the most likely reason for the presence of the Ffynnon Fair here must be its association with Plas Du. This small settlement certainly has a significant history, and although only a single building can be seen today, stony outcrops in the surrounding field hint at the remains of more buildings. It seems that there were around 15 cottages recorded in the 1851 census, though the settlement then was in a state of decay, George Borrow, in his 1862 book Wild Wales records his meeting with a motley collection of people here:
… I saw some grimy looking huts, which I supposed were those of colliers. At the door of the first I saw a girl, I spoke to her in Welsh, and found that she had little or none, I passed on and seeing the door of a cabin open, I looked in – and saw no adult person, but several grimy but chubby children, I spoke to them in English, and found they could only speak Welsh 
This story points to the fact that in the mid-19th century there were substantial ruins at Plas Du which had clearly seen much better days. It has been variously claimed, although I have been unable to find a definitive source, that the area used to have strong connections with Valle Crucis Abbey a few miles away, as the crow flies, near Llangollen and that Plas Du was originally developed as a monastery or halfway house operated by monks to provide shelter and accommodation for travellers between the Abbey and Chester. There is certainly evidence that an old track from here runs across Ruabon Mountain towards Valle Crucis, a tough five mile walk, and this would make a logical stopping point on such a route. This story could be the best explanation for the presence of a Ffynnon Fair here.
Today the well is firmly sealed off from passers by. It lies amid a ramshackle collection of sheds, protected by barbed wire and a “Trespassers will be prosecuted” sign. Probably much more in keeping with the Plas Du that George Borrow saw than that which welcomed monks of former times. The most likely location of the well is as shown below.
 Wrexham History website. https://www.wrexham-history.com/plas-du-newtown-mountain-penycae/ accessed 05.02.22
 Penycae Conservation Area Assessment and Management Plan. Wrexham County Council. 2011
Borrow, George. (1862) Wild Wales, Its People, Language and Scenery.