In looking for supportive evidence for the lineage your uncle drew up, you'll want to start with the person closest to you in time (in this case your grandmother). First document that her father was Hugh E. Masterson. Birth, marriage, and death records often name a person's parents. Family Search website may have her records transcribed, as well as supporting census information. If not, since you know where and when she was born, you can write to that county for a copy of her birth certificate.
Then move back in time, one person at a time.
Knowing more about the Mastersons in Ireland and Scotand may be interesting (I'm sure it is!) but does nothing to establish your heritage.