I have finally found the identity of my "Unknown" Monaghan that married Elizabeth McLaughlin in Baltimore, Maryland. He is Daniel G. Monaghan, born about 1841 in Ireland. He married Elizabeth McLaughlin on 12 Dec 1859 in Baltimore and he died about 1864 (probably due to the American Civil War). [Maryland State Archives Reel No. 23.14. Baltimore County Marriage Cards 1851-March 1, 1885. Issue Bk L.S.N 1851-1865, Folio 435. Minister: Lyman.]
I have also found one, and only one, passenger list showing a Daniel Monaghan. The ship "Alexander" arrived in Baltimore on 29 Aug 1853 from Liverpool. If the Daniel Monaghan on this list is my great great grandfather, then his family is as follows (all listed on the Alexander's passenger manifest):
Patrick Monaghan age 49 (b about 1804 Ireland), Margaret Monaghan age 19 (b about 1834 Ireland), James Monaghan age 14 (b about 1839 Ireland), and my great grandfather Daniel Monaghan age 12 (b about 1841).It appears that Patrick is traveling with his children but no wife.
Other than the marriage index card for "Monaghan, Danl G." and "McLaughlin, Elizabeth", I have found no other records for Daniel or for those family members listed as the Alexander's passengers. Does this family sound familiar to anybody else out there?