The key that the song might refer to the Clearances is in the last verse.
When they piped us all on board The lasses were singing But a tear came to my eye When the bell started ringing I will go, I will go. ... When we came back to the glen the winter was coming Our goods lay in the snow And our houses were burning I will go, I will go. ...
Otherwise the song is about going away to fight, which is the OTHER reason why there are more highlanders in the diaspora than in the highlands.