Martin gets younger and younger depending on which census you read. In 1900, he is 50 (born 1849); in 1880, he is 35 (born 1845); but he's the same age in the 1870 census (ie, born about 35); and he is 30 in the 1860 census which makes more sense if he immigrated by himself just before this time.The 1910 census has him aged 68 (born 1842) and immigrated 1864. The 1900 census indicates a year of immigration of 1863.
Where did you get the info about his sister's married names? If they are from marriage records in Ireland, you should be able to pinpoint the place of birth for Martin.
Martin's death certificate in 1916 gives his parents name as William Shaughnessy and Catherine Geraghty.