Shirley tells Richie that if he testifies that Vern helped him kill the Russian he can at least have the satisfaction of taking Vern down with him. However that would not work as a co-conspirator's testimony against a fellow conspirator is useless without corroborating evidence, and there was no evidence of Vern's involvement. The reasons for this are to prevent the very thing Shirley is suggesting, but also to prevent someone from trying to use false information and lying to try and get a reduced sentence in exchange for testifying.
Officer Murphey tells O'Reily that the term "Black Irish" refers to Irish that descended from survivors of a Spanish Armada wreck that washed ashore on Ireland in the 1500's. Which is a myth started in the late 1800's to try and explain why some Irish didn't have fare skin and red hair, which was what most Americans thought Irish were supposed to have. Irish in America faced almost as much racism and discrimination as black people did, so when someone that was Irish had darker skin and black hair white people thought they were that way due to having black ancestors. The Irish wanted to separate themselves from that myth so they created their own about the Spanish armada, though it was loosely based on real events. There was a Spanish armada that went down off the coast of Ireland in 1588, but very few survivors reached the shore, even fewer survived long term, certainly not nearly enough to effect the island's gene pool.