The Manx law of Heirship is a wide topic by itself. It is supposed to have Norse origins, but to put it simply, the original medieval law said that a purchased quarterland could be left by will, and it remained a "chattel" and could be left by will until it passed three descents. Once the three descents had taken place, it became "realty" or "heirship" and no owner could affect its ownership by will.
A man or woman's eldest son, or the child of the deceased eldest son, would automatically inherit. If a landowner had only daughters, then the eldest was the "heiress" as if she was the eldest son.
No property of heirship could be siezed to pay a common debt, although if a mortgage was secured on it then the land would be at risk.
During the course of the 18th century, the law was changed in favour of the heirs at law. The Deemsters and membership of the House of Keys were almost exclusively made up of eldest sons! Surprise!
The Manx were incredibly litigious, and almost inevitably any attempt to interfere with the descent of a farm would be met with a law-suit at Common Law. These often dragged on for years. Landowners were, however, aware of various methods for getting round the law. The most simple was the "sale and re-sale" where you sold all your property to a trusted friend for "valuable consideration" and a few (preferably years) later, if you survived, you bought it back for a bit more. This converted the land back to a chattel, and you could leave it by will. Alternatively, if you didn't survive, or didn't need to occupy the land any more, the ("bare") trustee, would sell the farm to the favoured child/relative.
The reasons for interference could be varied;

1. The rightful heir had left the Island and prospered elsewhere and would sell out if they inherited
2. The landowner had more than enough land to endow two sons (perhaps 1 from a "second venture")
3. The landowner had seriously fallen out with the heir (perhaps marrying beneath them etc) and wanted to cut him out
4. The heir at law was already a wealthy landowner and it was preferred to prevent the merger of a smaller holding with the larger
5. There might be a wish to keep the farm in the same surname perhaps through marriage to a daughter

Cases where the ownership of holdings was diverted include

1. Kaighin of Ballacregga, Michael, where the heir prospered in America. I have never found how this was achieved - there may have been a simple deed of surrender.
2. Heywood of the Nunnery and Whitehouse. The heir was sidestepped as he was a ne'er-do-well.
3. Much later the Cannells of Ballacarnane Michael diverted Shoughlaigue-e-Cain from the mother Jony Cain to the younger son Charles.

Many other cases.

If you are interested in when the law was alterd, I did a piece on it for the Proceedings of the NHAS.

NGC