As Malcolm mentioned above - there is no evidence that supports the claim that this is a song about the Clearances.
There is a long tradition, however, on Skye and elsewhere, of service to the King. For a long time, the call to duty from the Chief was all that was needed to turnout the clansmen. By the late 1500's, the call was also sent out among the clans for service to the king. While the king in Sterling had little authority over the highlands and islands, the concept of a king was held in very high esteem indeed.
By the time of James VI, most of the clans had submitted to Royal Authority. Thus, when the call was made in person for men to join a regiment to fight for the King, many found the call too strong to ignore.
I'm not certain that the exact period matters all that much. In my mind's eye when singing this song, I see a young man being called to serve in one of the Scottish regiments serving in France for the House of Stewart, either against the Cromwellians or the House of Hanover. I'd chalked up the last verse (added by Ronnie) to returning home from the wars, with all that he'd left behind lost to him forever - whether in fact it is destroyed is immaterial. He comes back a changed man (as does everyone who has been in combat or battle) and still does not regret answering the call.
Now, back to work.