The intermixing of Europeans with African slaves produced new religious ideas. This happened openly in the 1600s as Irish indentured servants intermixed with Africans. It also happened through the master-slaver relationship. What it gave birth to was little known piece of American religious history.
Many of the kidnapped Africans were Muslim, and a hybrid form of the faith continued on the plantations, combining Islam, Protestant Christianity, and Hoodoo. Hybrid Islam survived on the isolated Georgia islands into the 1870s. In regard to Hoodoo, it means to “conjure.” It is a folk practice, mixing the Germanic-Swiss hexmeister from Lancaster County, Pennsylvania, with indigenous African, Islam, Native American and European folk magic.
If you study the “Slave Narratives” recorded in 1936-38; the former slaves describe the mixed practices. These are on www.ancestry.com under the database: “U.S., Interviews with Former Slaves, 1936-1938,” and elsewhere on the Internet. Georgia Presbyterian minister Rev. Charles Colcock, wrote a guide for missionaries going on the plantation. His The Religious Instruction of Negros in the United States (1842): http://docsouth.unc.edu/church/jones/jones.html provides amazing details as to what he observed. He noted the slaves took the stories of the Quran and transferred them over to the Bible seeing both religions as the same religious idea (see page 125), and this was 1842! It’s an otherworldly and bizarre read by our standards!
Concerning Christianity, Catholic slaves often would mix their faith with Louisiana Voodoo. In the Protestant South, the Baptist and Methodist denominations would dominate African American life. There they mixed Hoodoo into their faith. Voodoo and Hoodoo are very different.
Just be careful not limit what it means to be Irish to white and Christian. Irish identity may have become submerged into the African American experience, but the Irish contributed to the mixture of ideas and faith (Catholic, Protestant and folk religion).