Hello Kate, I'm also attemtping to follow this family, but from the Furlong side. I have both Pendergast and Furlong ancestors from Wexford and am trying to determine if there is a relationship on either side with this family.
I'm also unable to find where James Furlong and Margaret Pendergast were married, although I find them living in Fitchburg, MA on the 1855 Census of MA. Margaret's age is listed as 24Their oldest daughter Anna was born in Fitchburg in 1852, as was their son William(1854)the 1st (he dies as an infant apparently and another son was named William) and another son John Laurence (1856).I show the family living in Westford, MA (a break-off from Chelmsford) on the 1860s. James Furlong was naturalized in 1852 with a home address of Fitchburg.
ON the 1855 census of Fitchburg, the family appears to be living with or near to William and Cathering Furlong and their daughter Mary.
Sorry if any of this is old news. I'd love to hear what information you may have on James Furlong, if any.