do you have the Irish birth records from the parish registers for this family? a marriage for Thomas and Honora? When was Dennis born? When did they emigrate? What part of Ireland?
can you post that info if you have it?
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died in Ware, Massachusetts, 7 July 1884
Thomas McMahon, age 10y 4m, born in Ware, parents Michael, born in Ware, and Margaret, born in Chicopee
https://www.familysearch.org/ark:/61903/1:1:NWCD-MH6-----
married in Ware, Massachusetts, 31 December 1872
Michael McMahon, 21, a carpenter, born in Hardwick MA, parents: Thomas and Honora
Margaret Mcnane, 20 something, born in Chicopee, parents: Philip and Margaret
both living in Warren, MA, when married, 1st marriage for both, married by L G Gagnier/Gassonier ?, French Catholic priest
https://www.familysearch.org/ark:/61903/1:1:NWTW-NV4the marriage was registered in Warren, MA also, since at least one of the parties lived there. The dates on the Warren records vary, 27 Dec 1872, 31 Dec 1873, for example.
But they give the mothers' maiden names. Michael's mmn was "Malone". Margaret's mmm was "McGraph".
https://www.familysearch.org/ark:/61903/1:1:NW1F-BFNhttps://www.familysearch.org/ark:/61903/1:1:Q29P-89FDhttps://www.familysearch.org/ark:/61903/1:1:FHYW-8JV------------
There is a small tree, mainly for Michael and Margaret here:
https://www.familysearch.org/tree/person/details/M1TQ-D6HIt names a brother John Mcmahon, said to have been born in Ireland in 1844, and married in 1873 in Dundas, Ontario. John's parents' names on his marriage record are Thomas and Hanna Mcmahon, so maybe so, but these are all common names. Unless you can find John with his parents in US/Canada, I'd be slow to count him as a son.
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a possible daughter to Thomas and Honora, if you count Mahon and McMahon as the same name. her parents' given names and location are a match.
married in Ware, Massachusetts 24 December 1865
Catharine Mahon, 22, born in Ireland, parents: Thomas Mahon and Honora
James Ryan, 20, born in Ireland, parents: James Ryan and Catharine
both living in New Braintree when married
https://www.familysearch.org/ark:/61903/1:1:N4SP-MYV