Rí, or very commonly ríg (genitive), is an ancient Gaelic word meaning "King".[1] It is used in historical texts referring to the Irish and Scottish kings and those of similar rank. While the modern Irish word is exactly the same,[2] in modern Scottish Gaelic it is Rìgh,[3] apparently derived from the genitive. The word comes from the Proto-Indo-European *h₃rēǵs ("ruler, king"). Cognates include Sanskrit 'rā́jan' ("king") and Latin 'rex' ("king, ruler").
From The Wikipedia at Ri.
Now see my previous posting, again, if need be.