Complex sentence: On the other hand the Belfast to Liverpool ferry, British owned and British crewed, used the tricolour as a flag of convenience; so did the whalers of Christian Salvesen Shipping, to take advantage of the Irish whale quota.
Split into simpler sentences: On the other hand the Belfast to Liverpool ferry, British owned and British crewed sailed under the tricolour. Some British ships registered in Ireland and flew the tricolour, such as the whalers of Christian Salvesen Shipping to take advantage of the Irish whale quota.