Which company's payroll? The IT company or the recruiter's payroll ?
I'm wondering why they're siuggesting two options, first A than B, and not presenting a choice of A or B (as other UK recruiters do).
how long are you emigrating for ? If for good, then likely you will lose Irish residency and (I stand to be corrected here) can apply to lose it immediately. This may be of benefit to you as if you are going fdown the road of a UK company (you being the sole director/employee/shareholder) there is a UK tax saving if you get paid by dividends from your own company (n/a if you're employed by the recruiter); added to which there is a nice VAT saving available to you operating through your oown company (equivalent doesn't exist in Ireland). This saving (/extra income) is not available if employed through the recruiter.
MOst recruiters in the UK have tied themselves in with accountancy firms specialising in these small one-man-band companies so while it may appear they cost an arm and a leg, they are worth it (if you can't provide your set of accounts/UK VAT return etc)