Apologies if I've made delete-worthy errors below, this is my first time to post.

Edward (Ned) Kelly has written about various bog body finds across Ireland, and has linked them all together in a general theory of "Kingship and Sacrifice." The mainstay of his theory is found in An Archaeological Interpretation of Irish Iron Age Bog Bodies. The Archaeology of Violence: Interdisciplinary Approaches, 2, 232.

There is nothing on nipple-homage in that paper, and I haven't been able to find a specific written work where he makes that claim unfortunately.

It appears, from interviews at least, to be based on two references in old Irish texts and a curiosity of the Old Croughan man, one of the bog bodies. There is also, apparently, a tradition of breast-sucking from other ancient civilizations that has informed the theory (***No sources for this claim, from McCarthy's translation of St. Patrick's Confessions, have been found)

1. The Old Croughan man is missing his nipples. Some think they were removed with purpose. Others reckon they disintegrated in the bog.

2. In the Confessions of St. Patrick, the saint himself reports being told to suck the nipples of a sailor. From McCarthy's translation "they began to say to me: “Come – we’ll trust you. Prove you’re our friend in any way you wish.” That day, I refused to suck their breasts, because of my reverence for God."

3. In the Saga of Fergus Mac Leti, from Binchy’s translation, the following episode occurs: This was the dwarf who sucked his, Fergus’s breasts and caught hold of his cheek as a token of asking quarter from him. ‘Why dost thou do that?’ said Fergus. ‘That,’ said the dwarf, ‘is one of the rules of fair combat with us.’ Hence comes today the custom of taking hold of men’s breasts and cheeks for the purpose of seeking quarter and making appeal to their honour.”

Kelly’s argument, as I understand it, is the following: The people buried in the bogs are Kings or nobles. They are missing their nipples. Texts tell us that nipple sucking and submission are related. Therefore Nipple-sucking must have been a part of Irish kingship. Remember that I haven't been able to find a clear statement of this argument in any paper, so there may be key parts that I'm leaving out.

The major criticisms of this theory are first that it’s not certain that the Old Croghan man's nipples were removed with clear purpose. Second, St. Patrick is a bad source for descriptions of the pre-Christian Irish. Third, the episode from the Saga of Fergus Mac Leti can be seen as a joke, or a parody. Mac Leti, who would be analogous to the reader (or hearer) of the saga, is shocked, meaning nipple-sucking may not have been widely practiced, at least not contemporaneously.

You’re free to make up your own mind about whether or not the idea is believable. There is some evidence for it, though it rests on fairly shaky ground. At the end of the day it’s funny, which is why QI and the National Geographic keep repeating it.