Name is William M. in 1900 Census. Also shown as Michael W. Cassidy on son Thomas’s marriage record in 1909 and on Loretta’s birth record 1893.

Their residence in 1900 is 21 Fulton Street, later Lawley Street, in the Port Norfolk, Neponset, section of Dorchester, Massachusetts. Five children born in England, one in New York and three in Massachusetts. He works for a railroad apparently.

1900 census:
William M Cassidy
Hellen Cassidy
child Mary A Cassidy
child Sarah A Cassidy
child Thomas E Cassidy
child Patrick Cassidy
child James Cassidy
child Michael Cassidy
child John W Cassidy
child Loretta Cassidy
child Stella Cassidy

1907 directory has William M. living at 19 Exchange Street, Dorchester and working as a switchman.
1916 directory has William, a signalman, living at 19 Mapes Street - apparently the new name for Exchange Street; sons John, a signalman, and James, a laborer, also at this address.