Thanks to all for the interest and help.....
My Gr. grandfather, Thomas Mitchell, born to Thomas and Elizabeth Dunn was born in the Longwood area of Meath in 1845 (D. 1890).... I can find no siblings... he emigrated to the US in the 1860's to NYNY (with a Carr cousin) where he married Bridget Naynor (1846-1898) in the 1870's......... they had 5+ births.... one who survived (obviously) was my grandfather, James Mitchell.... who later married Ellen Costello from Cornamagh, Athlone. I have recently been in touch with a Mitchell from the Longwood area..... any info is greatly appreciated.....