When researching within the areas of the British Isles it needs to be remembered that prior to 1776, the Americas were a property of England and therefore no immigration papers existed nor any Oath of Allegiance. Also, many ships coming from Germany stopped in England and English persons also boarded for America so they are later listed as having departed from England when they were actually German ships..by the same token, English, Irish and Scots were sometimes found on German named ships.The Irish people sometimes as a result of religious persecution fled to Scotland or England and therefore it is misleading just where they are found leaving from.
Irish Family History Foundation. The Irish Ancestral Research Foundation Chalkey's 3 Volume Set on Irish-Scot Settlements in Virginia(excellent resource)