Travel to Ireland while waiting for BRP?

[Purposive]

I was recently granted entry clearance (on a Tier 2 visa), and I'm planning to clear the border just after Christmas. I will need to self-isolate somewhere when I arrive. My girlfriend lives in Dublin, so I am considering the possibility of traveling immediately onward to her after clearing the UK border, so I don't have to isolate alone. (She has to remain in Dublin for work reasons.) This would of course mean traveling prior to receiving my BRP, which I would retrieve later, after self-isolating (from the UK post office that I chose during the application process).

Would this be permitted, or would it violate some policy to remain within the UK until you have the BRP in-hand? Given the permeability of the UK-Ireland border, I thought I should seek clarification. Any advice would be much appreciated.