I am no historian, but I suspect folk songs are often fairly unreliable as historical sources, at least without additional evidence. Firstly, even impartial and entirely honest witnesses may report the same event differently, as no once can see the whole. Creators of folk songs may not be impartial, they may be prejudiced because of their social class or position, because of more overt political opinions, or because they have a personal interest in the event or people involved.

Secondly, the overriding imperative of a song is to tell a good story, so if facts have to be changed to achieve that, or simply to make a rhyme, that's artistically acceptable.

In the case of Lord Leitrim, there seems to be some uncertainty over the motives for his murder and it seems more likely to have been political, but the allegations of 'droit du seigneur' seem to be unfounded. However it's a good story to tell about an unpopular landlord (today it would be alleged he was a paedo) and no doubt the audience were more than willing to believe it. To accept the ballad at face value is dangerous, as we cannot know without additional evidence whether it simply reflects local rumours, may even be the source of them, or is in fact the truth, or a partial version of it

That's not to say folk songs cannot play a part, they may contain some truth, and they may show how an event was perceived by the public. However they cannot be treated as reliable evidence.