COSTELLO-GEHERTY, unknown parish

I know one section of my ancestors are from County Galway, but I don't know which parish. My entire Irish family settled in Belfast, Allegany county, New York, USA, some coming through Canada. I don't know the Counties for many of them, but the COSTELLO's if I remember put it on their gravestones. Direct line includes Marcus COSTELLO-Mary GEHERTY>Jane COSTELLO-Andrew COUGHLIN>Mark COUGHLIN. Most residents of this area were farmers or dairy farmers, as cheese-making is still very popular here.

The information I have gathered from US sources...

Marcus COSTELLO, b. 06 Apr 1814 County Galway-d. 13 Feb 1892 Belfast, Allegany, New York, USA; son of William COSTELLO; brother, Thomas COSTELLO (1822 Ireland-1867 Belfast, NY) who married Elizabeth KING (1821 Ireland-1882 NY).

married (assumed while still in Ireland) Mary GEHERTY (seen a few spellings), b. 25 Mar 1814 County Galway-d. 25 Sep 1889 Belfast, Allegany, New York, USA

known children:

1. John COSTELLO, b. 1836 Ireland

2. Timothy COSTELLO, b. 12 Feb 1838 County Galway-d. 08 Dec 1889 Belfast, Allegany, NY, USA, married Maria MURRAY (b. 1841 Ireland-1899 Belfast, NY, USA)

3. Margaret COSTELLO, b. 01 Apr 1840 Schenectady?, New York, d. 04 Apr 1856, Belfast, Allegany, NY. Unsure of birthplace & don't remember why i have this listed?

4. Maria COSTELLO, b. 1843, d. 01 Jan 1922 probably Allegany county, NY; married Thomas BURNS.

5. Jane COSTELLO, b. 28 Oct 1846, Friendship, Allegany, NY, d. 18 Jan 1921 Belfast, Allegany, NY; married Andrew COUGHLIN*** (b. 16 May 1837 Rochester, Monroe, NY, d. 08 Oct 1901 Belfast, Allegany, NY); children: Mary A. (1866), Catherine (1868), James (1870), Elizabeth E. (1871), Andrew J. (1874), Margaret E. (1876), Emma J. (1879), Mark V. (1881), Alfred G. (1883) & Frederick J. (1888).

6. Katherine COSTELLO, b. 12 Feb 1850 Belfast, Allegany, NY, d. 11 Jan 1931, Belfast, Allegany, NY; married Michael BURKE (b. 1849 Portage?, NY-d. 1941 Belfast, NY), son of John BURKE (1812-1890) & Hanora NEARY (1818-1906) of County Mayo.

***Andrew COUGHLIN was son of James COUGHLIN (1803-1876) & Catherine HOWE (1805-1877) both of Ireland, but I do not know the County, if there was any evidence they might also be from Galway, it would be appreciated! Andrew had brothers: Michael (1833 Ireland-1914), Thomas (1835-1893), James (1842 NY-1916) & Francis (1844 Belfast, NY-1923).

Thank you for any information, etc.! It is very appreciative.