My brick wall - where are they from?

O'Connor, Cashman, Reardon, and Cronin

I have researched US Censuses as well as immigration, Tithe, and Griffith's records in Ireland, but with such common given and surnames it has been frustrating trying to find them in Ireland without definitive dates and locations in Ireland.  The saddest part of my research - I have not found any photographs of any of my great grandparents.  Any help would be sincerely appreciated for any information or leads to identify where my great grandparents originated from in Ireland.  Briefly, the following is where I am at in my research so far.

O'Connor or Connor

In USA records they are known as Edward O'Connor (1840-1919) and Julia (Cashman or possibly Costigan) O'Connor (1844/45-1917), with an occasional Connor surname listed.  They are my paternal great grandparents.

They immigrated from County Cork, Edmund in 1864 or 66 and Julia in 1866 as listed in US Censuses.  Family lore indicates they stayed in New Jersey for about 6 months, where first son John was born in 1866, prior to continuing to Detroit michigan.  As of this posting no reliable immigration records have been found.

Edward O'Connor [known as Edmund Connor in records found in County Cork] married Julia with the surname Costen or Costine on 28 Sept 1865.  Edmund Connor was born in 1840, Ballytrim Cloyne County Cork and Julia about 1844/45 in Ireland, presumably County Cork.  Except for the given name Julia, no reliable records have been found for her or her parentage.  9 of 10 children were born in Michigan in the USA except for the one in New Jersey.  Edmund is the son of Timothy Connor born 1803 Ballytrim Cloyne County Cork and Johanna Dwyer born about 1808 in Ireland, presumably County Cork.  I have not found any records for Johanna Dwyer.  Timothy's father is William Connor who married Mary Hankard 02 Mar 1794 Cloyne, Churchtown Parish, County Cork. That is what I have found so far without a definitive place to locate Edmund and Julia in County Cork prior to immigration.  Autosomal DNA tests have identified first to secound O'Connor cousins in the USA, but beyond this none of us have been able to find a genealogical link to a place in Ireland.  I did a Y-DNA test for O'Connor and found 4 people listed, but with 4 mutations between 2 of us and 9 or 10 mutations for the other two - not much help so far.

Reardon or Riordan

In USA records they are known as Timothy Reardon (1844-1891) and Hannah (Cronin) Reardon (1855-1921) my maternal great grandparents. 

As listed in US Censuses Timothy Reardon and Hannah (Cronin) Reardon immigrated in 1873 or 75.  I have no identifiable records for their births in Ireland (could be County Cork or Kerry) or when or where they married.  The Death certificate for Hannah, filled out by her daughter, lists Daniel as her father.  As of this posting no reliable parentage, birth, marriage or immigration records have been found for either of them.  The Reardon surname could also be Riordan.  Autosomal DNA tests have identifyed many Reardon cousins in the USA, but none of us have been able to find the genealogical link to each other beyond fourth cousins in the USA or to a place in Ireland.