The Lebor na gCeart or the Book of Rights is a 12th century that details customs and practices of Irish nobility in the Middle Ages. The text outlines the rights of all the monarchs in Ireland at the time and the revenue due them.
Myles Dillon describes the text:
the book of rights purports to record the rights of the Irish Kings, the King of Ireland, the provincial kings and the stipends due from the King of Ireland to the provincial kings... as in the book of Invasions the record is put into verse so the text consists, for the most part, of a collection of poems.
Lebor na Cert: The Book of Rights. Dillon, Myles. Dublin: Irish Texts Society, 1962.