After extensive research, partly expanded further by the help of what I deem to be web scraping through AI tools to pull research material (rather than solely using it to determinate substance and value of data), there is a phenomenal interaction transmitted hiterto regarding the Gaels and Frisians, in part through collaboration of Anglo-Saxon contemporary peers.
In Ireland, these Anglo-Saxons founded and maintained a monastery (Rath Melsigi) that focused their faith based on the model of the Irish connections to, for all intents and purposes, world building based on the ideals of what the Irish monasteries had already developed.
A student of this cross cultural development, named Willibrord (lived circa 658-739), of Northumbrian origin, went as one of 12 total monks on a mission to convert the Frisians to the faith, and later became the first bishop of the Diocese of Utrecht.
The Frisian king of the period was Radbod (Redbad) whom was clearly the perennial warrior chief of the era, and set on a path to unify much of the territory under one kingdom, fighting with continental contemporaries.
Later, one of Radbod’s descendants, Saint Radbod, was a subsequent bishop of Utrecht.
I will further link this post (as I am gearing to finish the OP, especially with this content as a focal point for the Gaelic and Frisian connection), with the relevant data in ensuing periods.
However, I point out that in my research, there was another piece of knowledge that was a potential caveat to the Irish-Northumbrian mission to Frisia, that being how Radbod was in fact a raider of Southern Ireland (and the mission was in part a response for the raids).
This needs to be researched further, although it is apparent the mission occurred around the year 690.
Gaelic Chronology
Re: Gaelic Chronology
The original sources about Willibrord and the Radbods do not appear authentic to me, but rather fiction from the early years of the printing press: 15th or 16th century — in the best cases (somewhat) based on real events and names, but placed too far back in time. The same goes for sources about Antiquity and the Romans. I am still in the middle of a radical mental reorganisation, but all starts to make more sense already and there does not seem to be a way back into the old paradigm.
It's good practice to ask what is the oldest source about X; when, where and by whom was it discovered; first referred to; first published in print. You will discover that the answers are either vague or it's always the same network: church, ‘Latin schools’, universities, so-called Humanists (and their successors). Aiming to steal and distort the history of the conquered. It does not make these sources completely worthless. Once you recognise the patterns, it becomes more easy to separate the wheat from the chaff.
It's good practice to ask what is the oldest source about X; when, where and by whom was it discovered; first referred to; first published in print. You will discover that the answers are either vague or it's always the same network: church, ‘Latin schools’, universities, so-called Humanists (and their successors). Aiming to steal and distort the history of the conquered. It does not make these sources completely worthless. Once you recognise the patterns, it becomes more easy to separate the wheat from the chaff.