Some of this you may have already see but am posting it as it gives important biographical details about John Martin of Moneymore, his 2 marriages and the children from those marriages-
"... Jane Martin was born in Moneymore townland, by Maghera town, Co. Londonderry, Ireland, 15 January 1815, her parents being John Martin and his wife Jane Shiels. Jane had an older brother, Robert, born 1809 and a younger sister, Rebecca, born 1817. Her mother died in 1817 and in 1818 her father married Sarah Espie who had children: Edward born 1819, John born 1822, Thomas born 1823, Sarah born 1825, Margaret born 1828, Joseph born 1831, William born 1832 and James born 1834.
John Martin and his second wife Sarah both died in 1838, after which the family, on the advice of their uncle Dr Robert Espie, were persuaded to emigrate. ..."
http://homepages.ihug.co.nz/~Smckelvey/pre1848/duthie.htm