I'm also trying to trace my Holleran line, but I don't know, with certainty, that they came from Cork.
My 2nd great grandfather Thomas Holleran arrived New York in 1850, with his future wife, Margaret A Reily. They married 1851 in Jersey City, NJ and had 5 known children: John, Maria Ann, Margaret, Catherine Veronica (my GGM) and Elizabeth.
When Elizabeth married in 1899, in Kearny NJ, there was a note on her R C marriage record of a dispensation for the marriage of cousins. She married Peter Gilmore, son of John Gilmore and Mary Britt(on). Therefore, there is a known connection between the 2 families, but I don't know yet from which side the cousinship stems.
When my GGM was baptized in 1860, in Jersey City, her godmother/sponsor was listed as Bridget Holleran. Unknown is she is a sister, cousin, aunt or in-law.
If any of this sounds like a connect, please let me know. I have marriage records for all the daughters. I lose John after the 1870 census. Thomas disappears from records about 1863 - last sighting is his Civil War draft registration, but no evidence found that he served or died in the War.
Margaret Reilly Holleran died 1882 in New Jersey, but her death certificate does not list the names of parents.
I have death records for Catherine, (wife of Joseph Patrick Hughes) in 1933, and Elizabeth, in 1939. I have approximate dates of death only for Margaret (married Charles A Drew) and Maria (married James F Ryan).
Hope we may be able to help each other.