I would guess that some of this, this, and the immediate history of the earls of Pembroke. (Let's see what I can repiece in two minutes. William the Marshal married Isobel, daughter of Strongbow and granddaughter on other side of deposed king of Leinster; after son Richard died and his Welsh alliance with him, Richard's brother Gilbert acceded, and he'd married a daughter of the Scottish king; these deaths occurred in quick succession, but mad consolidation/expansion of both territory and bureaucratic record-keeping occurred as well.) There is also this--speaking of bureaucratic consolidation!--which I hadn't known. Those mad increases in record-keeping made it easier to keep mercenaries happy (paid/fed).