In Appendix 2, Southern Reporters, in The South Reports the Civil War, by J. Cutler Andrews, "Nemo" is identified as L.H. Mathews, a writer for the Pensacola Observer. On p. 62, Mathews is cited as the reporter charged by Braxton Bragg with publishing information which tipped off the enemy about Bragg's intention of attacking Fort Pickens on April 12, 1861. Mathews was acquitted of being a spy and was released from prison. In refuting allegations that he was a Northern sympathizer, Mathews wrote the editor of the Montgomery Advertiser saying he was an Irish immigrant who had lived in the South since 1848 and had been employed by the Pensacola Observer for five years. Mathews also wrote that he had been accused repeatedly as being an ultraist on Southern Rights. A correspondent to the Memphis Appeal wrote on 25 May 1861 that Mathews had fled Ireland in 1848 because of his political views and described Mathews as "a stout partisan for the land of his adoption, the birthland of his Louisiana wife." Don't know if this is the same NEMO you were looking for, but hopes it helps.