I was going through https://en.wikipedia.org/wiki/Medieval_demography to calculate the number of Gifted there should be in each tribunal, based on the "perhaps 1 in 10,000 from the core book, and came across something interesting- while most Tribunals seem to have far to few mages, Hibernia has too many- the demographics indicate a population of roughly 600,000 which would be about 60 Gifted persons. The sourcebook also states that at least half of these are mages of native traditions, and that the native magi must supply apprentices to the order at what comes to 1/year, which would not work out unless the average lifespan was 30 years or less including for the Gifted. Above and beyond this the book defines covenants with a total of 48 mages of the Order of Hermes in them, before any player covenants, 13 of which are English, meaning 35 are native. So apparently Hibernia not only has overly abundant magical resources, it also had overly abundant Gifted.
I've always believed that the Gift was more likely to erupt in somebody growing up in a magic-rich environment. It's not nature or nurture but nature and nurture with magic, I guess.
It essentially a frontier for Magi. Wide open spaces, lots of vis. Attracts magi from Rome, Normandy that are starved for vis.
It's possible, I suppose, that Hibernia has a higher proportion of people with magical blood running in their veins than is typical (due to interbreeding with the Fir Bolg and Fomhóraigh) and this might result in a higher than normal rate of gifted people? I seem to recall somewhere in the Hibernia book it mentioning a certain part of Connaught that produced so many gifted people as to be remarkable even by Hibernian standards, but that the treaty prevents magi from investigating it.
I reread the Connacht section, it mentions abundant vis, and especially extraordinary vis, and that hedge wizards thrive, but nothing about an exceptionally high amount of Gifted. Also while I understand that it is a frontier with magi coming in, the number of specifically identified native magi in the tribunal book is higher than what should be possible. Now admittedly there could simply 8be* a higher number of Gifted, or that the estimation of Gifted is low in the core book and more survive to adulthood in Hibernia due to the intercession of the Druids, but the fact is that the anomaly remains unexplained.
I did some ctrl+f-ing through the book and found what I was thinking of, it's not in the Connaught section but in the Cioll Trí section later in the book:
"Túath Saidhbh has an unusual number of Gifted members, and always has. Its members wish to have as little to do with each other as possible, and there is no common culture within this túath. They draw lots for the position of ceannaire, seeing it as an onerous chore. Magi would dearly like to investigate why the region inhabited by Túath Saidhbh produces so many individuals with The Gift, but they are forbidden from Connacht by the Treaty even if they grew up there."