According to a quick search, "McMaster" isn't in the top 20 or even 50 most common Irish surnames. It's not in the top 100 Scottish surnames either, "Mc" being more Scottish than Irish, I would think.
nrscotland. gov.uk/statistics-and-data/statistics/statistics-by-theme/vital-events/births/popular-names/archive/100-most-common-surnames
I'm not trying to make a federal case here, but I wonder if some politicians take stage names like in Hollywood.