You should try to determine the parish, presumably RC, where Frank and Ellen raised their family. Griffith's was conducted in Monaghan in 1858-60. You might check the Griffith's index on failteromhat.com looking for Frank, James, and/or Peter. A quick glance suggests Clontibret or Donaghmoyne for starters. Their filmed records begin in 1861 and 1863, respectively, possibly early enough to show later marriages of some of the Cassidy children, and those marriages often show the parents' names. Diane