I think claiming sovereignity over a small island in an unimportant location wouldn't raise too many eyebrows, but if the the size of Ireland than it's essentially a kingdom-in-the-making and unless it's hidden magically or somesuch mundane kings would want to conquer it. So it would be mighty difficult to hold to it as magi lords without the dispute spilling over to the right of magi to be magi-kings and claim land in Europe. Which is a fine way to go for a saga.
I once had a City of Mages, called Gormenghast and inspired by the namesake book. It was hidden in a regio, however. It was partly a result of thinking about whether a city could be self-sufficient with magic.
The city had huge silos, that were enchanted with Ring-duration anti-spoiling magic and were filled yearly with permanent food and raw resources created by Momentary Ritual spells. Part of the duties of the magi-lords that ruled the city were to learn, Master, and cast said spells. I found that this was still quite difficult, as (a) they needed quite a lot of such spells, (b) it provided little variety, (c) it required quite a lot of raw vis that they needed to import from outside the city anyway. The result was that trade into the city persisted, and the rituals were cast but sporadically and only for some key ingredients.
The city also imported water from a nearby lake through an Arcane-Connection fed evelasting fountain. This required a pact with the lake's spirit. An elaborate caste of "Water Engineers" covenfolk developed in the city, constructing and maintaining a complicated system of canals, bath, and fountians fed from this main water source.
Garbage and water disposal was taken care of with a Perdo pit. Making the city exceptionally clean and healthy.
The ruling council of magi also provided the city with some city lights, and a central magical floating Great Dance Floor used as both a market and an arena for public events of all sorts.
Crucial for the city's self-sufficiency, was the development of an assortment of Guilds. These competed in yearly Contests, with the magi gifting the winning guild with magic items that improved their craft. This structure led to a wide structure of citizens quite loyal to the magi and with extremely high Craft and Finesse scores.
Perhaps the most important contribution of the ruling magi, however, was the Chamber of Truth. Where the judges of the city presided; or the ruling magi, in rare cases. This provided a lot of good law and order to the city.
Finally, there was the Palace, which was really a covenant of 6 magi. Well, 6 ruling magi, and perhaps some others. It has other features, such the Tower of Birds that was the air wizard's laboratory, and so on.