I've asked for this standing stone to be included on the database on the basis that the Edderton Stone (a near-ish neighbour, as the crow flies) has been included (listed at http://www.themodernantiquarian.com/site/5248/edderton.html). Both stones are generally thought to be Pictish or later carved stones, however both could conceivably be earlier standing stones which have been later inscribed and reused.
St Demhan's Cross stands not far south of the A949 Clashmore (Dornoch) to Bonar Bridge road in a field which slopes down toward the Dornoch Firth. The site is marked with a brown tourist sign, which you'll spot just to the side of the Creich burial ground. There is a layby outside the burial ground gate, which you can park in. A stile and some steps take you down to the stone, at the East end of the graveyard boundary wall.
The stone is listed on the HER database (linked) as being "Early Medieval - 561 AD to 1057 AD" and is generally considered to be a Pictish carved symbol stone.
It is included in the Highland Council's Pictish Trail tourist information leaflet, which states…
"A rough, unhewn standing stone, bearing an incised Celtic cross. Like the Clach Biorach at Edderton this could be a reused Bronze Age standing stone. The carving probably dates to the 9th or 10th century AD. This may have been used as a preaching cross rather than a grave-marker. The fair of St Devenic (or Demhan) was still held here in 1630. "
The carved (north facing) face of the stone is rather smoother and flatter than the other sides. Which would obviously have been convenient if the carving was made later.
To me, compared to some of the other Pictish carved stones in the area, the carving looks rather crude and shallow - which makes me feel it's more likely to be a reused stone, carved in situ (perhaps where the carver would find it more challenging to work and produce a finessed and elaborate piece of carving). I imagine if a more modern graveyard was sited next to an ancient standing stone then the best way to absorb it into the Christian site would be to make it into a cross. If the stone was already being used as a marker for a meeting place, this would also cause less confusion than taking it down altogether perhaps? But that's just personal conjecture!