Did you actually find a birth record for your grandmother in Monson, MA?
I found the Finnegan/Haggarty marriage in Monson and I found Mary A. (Haggerty) Finnigan's Death Cert. in Monson.
I also found a couple of things that 'could be' your family in Berkshire County...looks like your grandmother's marriage information.Also, a birth record in Pittsfield that surely looks like it 'could be' your grandmother..Lizzie Finnegan, born Sept 4, daughter of Patrick and Lizzie.Patrick born 'Munson', Lizzie born Deposit.
And, there's a 1900 census record in Pittsfield that 'could be' your family.
Let me know if you actually did find a birth record in Monson..if so, the information that I found would probably be the wrong family. Carolyn