I suspect that Thomas Brennan who married Catherine Comerford in 1833 is either a brother or close connection to my 3rd great-grandfather, Patrick Brennan 1813-1885.
Reasons:
Patrick lived in Co Carlow very close to the border of Kilkenny. His eldest daughter was born in Lacken in 1837. His marriage to Mary Ann Jordan (1835) and Maggie's baptism are in the records of St. Lazarian RC in Leighlinbridge.
The marriage of Patrick's brother John (1809-1870s) and Mary Rice in 1836 and the baptisms of their children Margaret, Catherine, Mary, and James are at the same place.
The marriage of Patrick's sister Catherine (1812-1869) to John Roach in 1833 and the baptisms of 8 children are also at the same place.
But Thomas (~1805-1860s) and Catherine's marriage and 2 oldest children aren't recorded there.
In 1842 Thomas and Catherine are in Beckwith, Ontario, 3 lots down from Patrick (who shares a lot with his brother in law Martin Jordan).
In 1851 Patrick Brennan and Thomas Brennan have both moved their families to Grey County, south of Owen Sound.
John and Mary (Rice) Brennan are on the lot Thomas was one in 1842; next door is Catherine Brennan Roach, who has been widowed. (bottom left of red rectangle)
http://brennan-families.com/beckwith.jpgIn 1861 William Brennan is on Con 2, lot 67 of Holland Twp, and Thomas is there listed in the Directory of Grey Co of 1865-1866. (just to the right and slighly above center)
http://brennan-families.com/dornoch.jpgCatherine is widowed and living in Chicago in 1867, 1869, 1874, address 230 N. Halsted. But on the 1870 census, daughter Margaret and her husband Edward Comerford are living there, as is Margaret's yougest sister, Ellen. (I don't know if Edward is related to Catherine or not, but Margaret Brennan Comerford's death record lists her mother as Catherine Comerford and lists her own birthplace as Kilkenny)
In 1880 Catherine is at 104 Liberty in Chicago with sons John and Thomas, divorced daughter Ellen, and Ellen's kids.
I'm nto positive it's Catherine's Thomas is my ggg gf's brother, but it's pretty suggestive.
Chris