There were 4 children of John Cogan and Mary Sheridan. The all immigrated to the US. The only information that I have found as to the year that they immigrated was attached to the youngest of the 4, Thomas Cogan. There was a biographical write up in the county on him. It says that the parents stayed in Ireland but that Thomas who was only 14 years old at the time, immigrated to the US with an older brother. That would be either John Cogan (Jr) or Patrick Cogan, I suppose. Patrick was born 1822 and John 1826, so both of them were old enough I suppose to leave. They immigrated in 1847 and Thomas spent some time driving teams on the canals in Delaware and Hudson Rivers in the northeastern US. They went to Ohio in 1850. I wonder if you know some birth dates of your Cogans. I would think that it would be a strong possibility that these Cogans and Monaghans are all related, maybe siblings or cousins.
I'm happy to correspond with you on these families.