Then your grandfather would be Ol' Sean Thornton.
8 seconds sound clip from the The Quiet Man (1952) movie soundboard.
You can hear this line at 00:20:49 in the Blu-ray version of the movie.
Quote context
[...]
- I do not believe I heard your name, tall man.
- Mike. Michael Thornton. He used to live here, by the Mornin'.
- I just bought the place back. That's why I'm here.
- Then your grandfather would be Ol' Sean Thornton.
- Right.
- So it's himself you're named after.
- Well, now. That being the case, it is a pleasant evening and we will have a drink.
- There was a wild colonial boy, Jack Duggan was his name.
[...]