The general law infers contractual warranties as to quality as well as a duty of care to users. The standards are higher if selling to consumers as opposed to selling to another business. However, some modifications to the general law standards are allowable in written business-to-business contracts.
If the goods are defective, and (in a business-to-business context) in the absence of any explicit provisions dealing with this in a written contract, because the sale will have occurred within Northern Ireland, in the normal course the buyer will sue the seller in the Northern Ireland Courts i.e. where the contract occurred.
Even if the contract occurred in Ireland (e.g. where the seller from Northern Ireland sold the products in Ireland through a Sales Agent in Ireland) it may still be more appropriate to sue the seller in Northern Ireland as it would be easier to enforce a Northern Ireland judgment against a Northern Ireland seller.
There are, however, circumstances where it may only be possible to issue proceedings in Ireland where witnesses in Ireland are not compellable to attend Courts in Northern Ireland to prove the circumstances of the contract.