Searching for Thomas Regan and Margaret Sheehan in County Cork

I am searching for the Townland of my ancestors Thomas Regan and Margaret Sheehan Regan.   Below is the information that is known. I think that they may be from an area near Mallow.  My cousin will be in Ireland in a few weeks and would like help in researching this family.  Thank you in advance for any help you can give to us.

Birth dates and dates of immigration are approximates taken from census records and death certificates.

This family settled in the County of Onondaga, New York StateThe earliest census record found for any of this family  in Onondaga County is 1865

O'Regan/Regan/Reagan
Thomas Reagan and Margaret Sheehan

Oldest known ancestors are Thomas Regan born about 1800 and Margaret Sheehan born about 1810. 

These names were listed as parents on the death certificates of two of their children - Thomas and Catherine.
They were also found in census records living in the house of their son Jeremiah.

The 1865 New York State Census lists that Margaret was the mother to 10 children but we know of only 6.
 

Known children of Thomas and Margaret Regan:
 

Thomas Regan born about 1829 in Ireland d. 12/151919 age 90 m. Julia McCarthy d. 2/22/1902

1900 census lists immigration date as 1857 for both, years in US 43 , years married 45 suggesting that they were married in Ireland
 

Jeremiah Regan born ? d. 3/18/1869  married Ellen - possibly Flynn d. 2/8/1882

1880 US census shows one son Jerry 18 years of age born in Ireland
 

Patrick Regan b. 1835(from headstone) d. 9/10/1899 m. Bridget Gilvie b. 1840(from headstone) d. 4/20/1905

Married in St. Francis Church, Marcellus, NY 2/15/1863 buried St, Francis Cemetery

1900 Census lists immigration date as 1858 for Bridget
 

Mary Regan b. 3/1847 (1900 Census) d. 5/24/1912 m. John Radford

1900 Census lists year of immigration as 1860
 

Catherine(Kate) Regan  m. James Reagan b. 5/1844

1900 Census shows birth as 5/1848 but death certificate list 12/25/1849 died 4/27/1920 age 70

1900 Census lists immigration date for Catherine as 1865 and James as 1864  but 1910 lists 1866 for both

1910 Census lists years married as 40 which would indicate that they were married in Ireland based on immigration dates.
 

Daniel Regan b. abt 1856 no known spouse

died 6/29/1906 at age 50
 

We are descended from Patrick Regan and Bridget Gilvie and the family lore is that Patrick and family were from County Cork and that Bridget was from County Mayo.
A descendant of Thomas Regan and Julia McCarthy has been researching the McCarthy line and have found John McCarthy of Cloonbannin East to be their ancestor.
I found a gravestone in a local cemetery for John Regan  inscription reads: John Regan died July 2, 1902 aged 75 years  Anne Barry his wife died 8/14/1887  Born in Garryduff near Mallow County Cork Ireland.  I have not been able to connect this John to our Regan line.
The names Cronin and Purcell are often connected to the Regan name by marriage and as neighbors in census records.