Unfortunately for the story, it is very unlikely that enough of the sailors survived for their genes to be in the population visible today. A variation on this theme says they are descended from Spanish Moors who traded with people on the west coast of Ireland. The descendants of these slaves and black slaves from Africa live there to this day. A majority of Irish people who emigrated to America in the 18th century were Protestants from Ulster. Most of these, in turn, were descendants of settlers brought in from Scotland from the 17th century during the so called plantation of Ulster.
Black protestants are protestants who take their religion seriously.
There is some speculation as to where this expression comes from. It might also have something to do with the Royal Black Institution, a body for Ulster protestants similar to the Orange order. In many ways they can be seen as a separate ethnic group and they are often subject to ethnic prejudices and discrimination. Clover is used for large species and shamrock for small species. Shamrock, like clover, is common in Europe, not just in Ireland.