Hello from Ontario, Canada! I know exactly who you are speaking about when you refer to William Seamore Walsh. He was my mother-in-law's uncle, brother to her father, Freeman Walsh (born 1887, Mt. Tryon, P.E.I.) William was the third of 10 children of Patrick Walsh & Mary Thomas. Freeman was the 10th. Lately I have been corresponding with descendants of 2 other brothers, Sampson & Frederick too.
My mother-in-law, still living, is 75 now & spent a couple of years in childhood in Massachusetts. She remembers it quite well.
Can I help you any further with the Walshes? Sorry I don't know any more about the Cassidys.