Posted: Tue Dec 1, 2020 1:47am
With your Irish passport you are an EU citizen and as such for you visas do not apply or even exist, that's not to say however that the 90/180 day rule doesn't still apply, it does as it always has.
With your UK passport, both now and after 1.1.21, you are similarly subject to the 90/180 day rule.
With either to stay longer than that residency is the only lawful route.
That said after 1.1.21 for all but newly disenfranchised UK citizens, who will now be having their passports stamped both in and out, the freedom to stick two fingers up at the law and stay at their leisure will continue much as it has until now.
In theory at least I suppose it might be considered something of a loophole if you were to alternate between passports leaving the country before 90 days on one then immediately returning for up to 90 days on the other, and so on.
Whether immigration would pick up on a name in that circumstance I couldn't say.