Brehon (Irish: breitheamh - IPA brʑeçəvɤ or brʑeçuː) is a term for a historical arbitration, mediative and judicial role in Gaelic culture. Brehons were part of the system of Early Irish law, which was also simply called "Brehon law".
and the article goes on to say brehons were arbitrators, sort of judges, with the know-how to settle disputes at a high level. Whether this system is 'Irish' or a more widely applied 'information network' isn't stated. Judging by the confusion, perhaps due to their apparent relatedness, of saints [and indeed the strange case of the Rolls of Oleron] the latter seems likely.
So it may be that offshore and/or tidal islands were for professional pilots, people with detailed knowledge of local conditions.