Brennans (Brannans) documented as being born in County Mayo, Ireland in the 1861 England Census Records
The early years of the England Census records infrequently report the County of birth in Ireland, but notation of more than just the country becomes more frequent from the 1861 census on.
These records will be presented alphabetically by given name.
There was a John Brennan, lodger, 31 in Glouchestershire, Cheltenham who is listed as being born in Croghan, Ireland. There is a Croghan Ireland in County Mayo, County Roscommon, Tipperary and Kings County. He wasn't surrounded by individuals from either Mayo or Roscommon so I have not included him in this compilation.