Is the buyer or the seller responsible for completing customs declarations, transporting goods and paying duty?

Ireland and Northern Ireland: The responsibility for completing customs export and import declarations depends on what has been agreed in the contract of sale. International commercial terms, or "Incoterms", identify where the responsibility lies between the seller and buyer (e.g., for export declarations, import declarations, transport, insurance, import VAT and customs duty) and where risk passes from one to the other.