How African American families got their surnames is often misunderstood, affecting your ability to connect into your Irish ancestor. The popular notion is that the slaves took the surname of the last master.
You may find your family did not have a “slave name” at all. Often surnames were used for several generations within a family. It may also be your ancestors were not slaves. You might be descended from those bi-racial and tri-racial families from the 1600s; descendants of African men and Irish woman. This free segment of the population was more common than you might think.
The 1870 Census is the first federal schedule which lists the former slaves by their full names. This is a pivotal record. If you find your ancestors in the census schedules prior to 1870, then start asking some serious questions about what “free color” means.
In some families surnames came into use just like they did with the free population, from the father or mother. In slave families, surnames were often used, but not publically. The slave owner had little reason to know, use or care about slave surnames.
The surnames of slaves might signify a major event or person such as a favorite or first master. Often surnames were chosen for various other reasons; a political figure (Washington, Lincoln), a first name (David, John, George), a principle (Freeman, Love, Pride), an occupation (Carpenter, Mason), or a place. These were sentimental ways of forging an identity as a family unit apart from the brutality of slavery.
Remember, a family could have used an unofficial surname for generations, but the first recorded evidence might have been in the 1870 Census. This is your pivotal record in exploring the origins on your surname.