Yes, you will need an accountant - one versed in both jurisdictions' tax codes. (I've said it elsewhere but try to find one with AITI / CTA if looking here in the Republic or CIOT / CTA if in the North, there is another UK tax body to look out for, but the name escapes me right now)
I may be wrong in the following but confident enough to put it out there to be corrected - > Essentially you own a trade in the South but have a branch in the North. Just so happens this Branch contains 100% of all business assets/liabilities of the trade.
I can't recall provisions of the DTA, but v v v likely it is a credit, as opposed to an exemption, but certainly not a deduction as the case with other DTAs.