I don't know if there is any connection between your CROTTY-AHERN family and the one I have been tracing. Although Lismore is in Co. Waterford, it is only a few miles from the Cork border. I think it would not be unusual to find other Crotty-Ahern marriages in that area of Ireland. Do you know what area of Cork your family came from? Have you traced them in records there?
I updated my information on the Crotty Forum in November 2002, but I seem to have missed posting it here. So here is is.
I did find William CROTTY and his family in the US. The advertisement he placed in The Boston Pilot on 6 Feb 1869 must have worked, because William and his family were enumerated in the 1870 census in Paterson, Passaic County, New Jersey in the same dwelling as his brother-in-law Patrick CONDON and Patrick's wife Margaret AHERN! So they managed to find each other before June 1870. Their name was erroneously written COTTY in the 1870 census.
Here is the information given about the CROTTYs in that 1870 US census:
William - age 38, b. Ireland, laborer, US citizen Ellen - age 35, b. Ireland Margaret - age 15, b. England, works in silk mill Mary - age 12, b. England, works in silk mill John - age 8, b. England, attends school Bridget - age 6, b. England, attends school
Unfortunately, I could not locate this family in the 1880 US census, though they may still have been in Paterson. I still need to work on tracing them through Paterson city directories and vital records. I did find Ellen, widow of William, listed in the Paterson city directories in 1890-93, along with John CROTTY (a carpenter living at the same address as Ellen - probably her son). So they must have still been in the area. I found John again in the 1910 census, married to wife Sarah for 21 yrs. They had no children, but a niece Margaret Campbell (b. NJ, age 21) was living with them in 1910. I don't know how she was related.
I have no more information on other family members or on the family in Ireland at this time. Can you see a connection?