It is very wise in all instances to have a written contract, particularly in business-to-business contracts where there is more scope to modify the provisions of the general law. In cross border sales, it is particularly important to define such matters as when risk passes, the liabilities of Distributors or Sales Agents etc.
As a result of Brexit, it is not currently clear whether the EU Regulations governing civil and commercial contracts between member states will apply to Northern Ireland after March 2019. As a consequence, in order to avoid real commercial consequences (i.e. legal fees and delay) resulting from legal uncertainty as to which country’s courts have jurisdiction to resolve disputes post March 2019, in business-to-business sales you are strongly advised to contain an explicit jurisdiction clause within your terms and conditions when trading cross-border. It is highly likely that such a clause will be recognized by the courts in both jurisdictions. (In sales to consumers, such a clause will not be binding on the consumer.)