March law was a set of laws and customs obtaining in the border areas of the Lordship of Ireland during the Middle Ages.
It came into being in the late 13th century, when King Edward I of England drained resources from Ireland to fund his conquest of Wales and his wars in Scotland.
Since the two areas were often intermingled in the border regions, as in the Wicklow Mountains, the applicability and content of march law varied widely.
The Old English were permitted, under march law, to negotiate with Gaelic cattle rustlers, since the English authorities in Ireland were too weak to pursue such felons.
In the case of certain crimes, the criminal could be imprisoned by his family.