Hello,
We just found this out! On the death certificate for his daughter Bridget Dargan Condon it lists her name:

Catherine Brady b. abt 1796 and died before the 1870 census. She is missing in the 1870 census.

1870 Jamestown, Howard, IA census
John Dargan age 28 born Ireland
Mary age 21 WI
Kate age 3 WI
Thomas age 1 WI
John Dargan age 71 born Ireland
Pat Rooney age 25

Best,
Sarah