The Irish and Welsh chose to come here for the most part, most blacks (prior to 1865 at least) did not. Even the Irish used as indentured servants were not slaves- their children were not born indentured servants/slaves, they were free after seven years, and once free they had the same rights as other free whites. The children of slaves were born into lifelong servitude along with their parents. Slaves could not marry, learn to read, or testify on their own behalf.
The Irish and others had it hard in the United States, but it was not the same as slavery.
Actually, that 7-year figure isn't always accurate - some whites were brought to this country as "indentured servents" and it was for life - not just seven years.