In some cases, people who emigrate consciously decide "I will keep my native accent" or "I will fully adopt the accent of my new surroundings" (this particularly if their native accent leads to discrimination and disadvantages finding a job), and factors like age, class, "ear", and whether your native accent is looked down on will make it more or less likely for someone to try to modify their accent.

But accents are deeply linked to our sense of self, our identity. So the subconscious need to either retain your sense of belonging to your native culture or to adapt to your new one can tip the scales one way or the other. (Since we're talking about Kevin's accent in English, I'm just focusing on accent change for people who move somewhere where they speak the same language.)

To give an example: I have two friends who moved here from Spain as kids.

One was eleven, at this point she has lived here longer than she ever lived in Spain, but her accent has barely changed. She's very proud of her heritage, her entire extended family is still in Spain, she visits at least once a year, and she has said to me that she feels her accent makes her more interesting and helps break the ice when meeting new people. Modifying it was never a goal for her, in fact, retaining it makes it easier to slip back into her Spanish community when she visits, to not feel alienated from them.

The other one was thirteen. She moved here with her mom and siblings after her mom's divorce from her abusive step-father and left no extended family behind. The move was a fresh start for her, and starting high school in a new country as a very shy girl her main concern was fitting in. Her accent started to shift and soften in just a few months, and within a couple of years she had lost her Spanish accent almost entirely.

So, if you're team Kevin keeps his Irish accent / he code switches / it comes out when he's drunk or tired: it's Kevin's link to his mom and his childhood pre-Nest, his accent helps him maintain that connection to the part of his identity untouched by Riko. He clings to it regardless of the passing of time because it's the last part of him that is his and his alone, not amalgamated to the Ravens as a collective.

If you're team Kevin loses his Irish accent entirely: his survival depends on adapting quickly to the Ravens and their hive mentality, where being different, being an individual, is not only not appreciated but dangerous. You're not a good cog in the machine if you stand out, if you keep a grip on your sense of identity. So he probably loses the accent in record time; within the year he sounds just like most of the Ravens, his accent tucked away along with his past and his mum somewhere deep inside, where it's not a liability and where it remains only his. It becomes part of his idealized memory of his childhood, preserved in amber, not touched by his new reality.