Well as Eco says you can tell someone's a lunatice because sooner or later they bring up the subject of the Templars.I do not know of this song but I have some information that might help. I am unaware of a Roslin Castle in Orkney but there is a Roslyn Castle in Roslyn just outside Penicuik about ten miles South of Edinburgh. There is also a nearby Chapel, still intack, which is of much more interest than the castle ruins.
I believe that this is the castle of your song. Both the castle and the chapel belong to the Sinclair family and have done since the mid 15 Century. There wasn't a Templar battle and the property has never left the posession of the Sinclairs. However, the castle was sacked and ruined by Cromwells New Model Army although the chapel was spared possibly because of the blatant masonic imagary carved within.
The chapel itself displays the most exquisite stone carving. It really needs to be seen to be appreciated. There are Templar grave stones inside and there are many Templar connections.
The song you are looking for may be about the sacking of the castle by Cromwell and may have some Templar allegory within. Alternatively it may be about the disbanding and persecution of the Templars as Heretics in the early 14 Century.
I suggest you contact the existing owners of the chapel who may be able to help (I don't have the exact address but I'm sure I've given you enough to go on). I'd be interested in what you find.
Good hunting
Sandy