Table schema:
CREATE TABLE table_name_9 (tonnage VARCHAR, ship VARCHAR, builder VARCHAR, original_operator VARCHAR)

Question: What is the Tonnage that has builder AG Vulcan, and the original operator NDL, and the ship, Prinzess Irene?
SQL: SELECT tonnage FROM table_name_9 WHERE builder = "ag vulcan" AND original_operator = "ndl" AND ship = "prinzess irene"