I think it's entirely plausible…the Irish were not on the top of the social chain when they came to the U.S., just as African Americans, so there would not have been a class stigma involved.  Interracial children would probably identify themselves as being more black than white, which would mean that they would tend to marry African Americans rather than Caucasians.  Think about all the black people with Irish last names: Shaquille O'Neal, Eddie Murphy, Tracy McGrady of the NBA, Beyonce Knowles...so I'm not surprised.