It will be necessary to decide whether you propose to trade as a Sole Trader/Partnership or as a Limited Liability Company in Northern Ireland.
If you are operating via a company, regardless of whether the trading is conducted via a subsidiary or a permanent establishment, there is a requirement to register the entity with Companies House. If you choose to operate via a company, then you need to establish a company and register with Companies House using form IN01. If you choose to operate via a branch, then you need to register the branch with Companies House using form OS IN0I. These need to be submitted together with the necessary documentation. A charge applies. Annual accounts must also be submitted to Companies House in respect of subsidiaries.
Whether you choose to operate via a subsidiary company, a branch or as a sole trader, remember to register with HM Revenue & Customs on time to avoid late registration penalties.
If you choose to establish a presence in the UK, registration for VAT must be considered but you are entitled to the same registration thresholds in the UK as an ordinary UK business. If you exceed this threshold, you must register for VAT within 30 days.