My museum time today yielded the following:
Sucking a King's nipples was an ancient Irish form of submission. It rains a lot here (Dublin) and is rather chilly, so I would think the King would cover his chest. That means there must have been royal reception days when the King exposed his nipples in order to facilitate nipple sucking.
So much easier to just bow and kiss a ring.
As with all royalty, there were power games in the nipple hierarchy. Cutting off a royal descendant's nipples made him ineligible for kingship. Not as subtle as poison, but undeniable evidence of his unsuitability for a kingly role. No nips, game over.
A Celtic King was wedded to the Earth, and as her representative his nipples were important. His/her power is transferred to the grain. When it's harvested, his power is sacrificed. The Lord must die, Joseph Campbell said: "A God dies for his people so that they may live." The story repeats itself in multiple mythologies, legends and religions. But must the mortal king die in order to insure a successful harvest?
Human sacrifice was apparently a normal part of the Celtic rituals, especially of kings in hard times. "The king had great power but also great responsibility to ensure the prosperity of his people. Through his marriage on his inauguration to the goddess of the land, he was meant to guarantee her benevolence. He had to ensure the land was productive, so if the weather turned bad, or there was plague, cattle disease or losses in war, he was held personally responsible," said Ned Kelly, keeper of antiquities at the Irish National Museum.
His kingly role required him to keep nature and society in equilibrium. A little nipple sucking would surely increase his self-esteem and help him on his way.