Here he goeth speculating again.
The word turncoat could be used literally in Napoleonic times. A deserter only needed to turn the red (or green) outside of his coat inside to pass for a civilian, at least at a distance. Somewhere between Waterloo and the first world war, turncoat became an expression, used in a milder way than deserter, e.g. an elected politician changing party affiliation could be called a turncoat without the threat of a libel suit. Since it seems to be used as the equivalent of traitor here, I assume that it dates around 1815.
After 1815, the British army was evacuated from France. The soldiers who had married without permission (the lower ranks) were forced to leave their family (many in the army train) behind and to their own resources. Staying behind was considered desertion, punishable by death. On arrival, the ex-soldiers found there was no work for them. Demobilisation had caused a severe depression, as government expenditure diminished sharply. In theory, the wounded became wards of their parish. In practice, the parishes were overwhelmed and many invalids ended up begging also.
A wave of crime hit the UK. It was met with a wave of hangings, with due process suffering badly from public anger with the demobilised soldiers. The point is that Ireland was represented far beyond the size of its population among the common soldiers. They had run from hunger, misery and poverty in Ireland to serve in the British army, only to be reduced to human waste when their services were no longer needed and their accent gave them away at every turn. I suppose it wasn't the first or last time something like this happened, but in a climate where the Irish in England were a favourite minority to hate, I can only assume that this counterstamp represents the feeling of the angry, seething masses.
In that sense, you have acquired a vivid document explaining why Ireland eventually became an independent country. Nobody learned anything.
Peter