Hi Aileen
A Lynne Sisk is researching a James Brennan who was born in Castlecomer c1815-1820, the son of Martin and Mary. The family were stone masons and moved to New York.
Of couse it's possible Martin and John were brothers, but really it's just as likely they're not as it was such a common name (I should know - my great-grandmothers names were Smith, Brown, Williams and Wilson!)
I'll keep trying to establish any connections and break through our 'brick walls'...
CV-S