Early Gaelic law tracts, first written down in the ninth century reveal a society highly concerned with kinship, status, honour and the regulation of blood feuds.
King Robert Bruce cites common "customs", as well as language, as features which made the Scots and Irish one people.
In the twelfth century, and certainly in the thirteenth, strong continental legal influences began to have more effect, such as Canon law and various Anglo-Norman practices.
There also existed an official called the judex regis (i.e. "King's Brehon"), and perhaps this status was a way of ranking various orders of Gaelic lawmen.
For instance, Mormaer Causantín of Fife is styled judex magnus (i.e. great Brehon), and it seems that the Justiciarship of Scotia was just a further Latinisation/Normanisation of that position.
He supervised the activity and behaviour of royal sheriffs and sergeants, held courts and reported on these things to the king personally.
As English expanded westwards in the thirteenth century and after, Lothian came to include not only the core south-east, but also subordinated the sheriffs of Stirling, Lanark, Dumbarton and even Ayr.