"Leitrim was generally hated - by his tenants, by his fellow landlords and his fellow peers"

Yes, Slevin says that and all of the historians seem to agree on that. However, the specific accusation of droit de seigneur, none of the three historians seem to support. And that is the accusation that you claimed that a song provided support for.

Pursuing someone to bankruptcy might also provide a reason for someone to hate him.