Joe, thanks for the correction on the name spelling. To answer your first question, the belief that the McCreachery name is via the Old Judge book, p. 87.
"The final example is a rare portrait card of Deacon White. A fictitious name, McCreachery, is listed together with the title of manager for the Indianapolis club. Deacon White, the oldest player in the league, was apparently better suited to manage from the bench than play third base for the Detroit Wolverines."
To answer your second question, there are a lot of players with the prefix "Mc" in the OJ set. Obviously, they were not being slandered - those were their real names and they were actually Irish. But Deacon White was not Irish, his last name of "White" is obviously not Irish, and that is what makes it slander to change his name to an Irish one.
To call an Irish person a "Mick" is an ethnic slur. I would never use that language but you can see where it comes from. Personally, not only don't I have anything against the Irish, but it is a dream of mine to visit Ireland. From everything I had heard and seen it is a spectacularly beautiful country and the people are very nice. I also like Celtic music, which is really cool.
But in 19th Century America, not everyone did love the Irish. A lot of it is due to economic forces that make people feel they need to battle for resources, without which would threaten their livelihood. This makes them suspicious of outsiders. Even native-americans were called "foreigners" in the pre-war time period. How hypocritical is that?
