Rob, of the two Vicksburg companies in the 3rd Confederate Infantry, only the "Shamrock Guards", Company D, had a distinctly Irish character. The company was raised by Capt. John H. Crump, and was mustered into service on June 18, 1861. A comparison of the company roster with the 1860 Warren County census indicates that the company was recruited mainly among the Irish riverboat men and day laborers of Vicksburg.

The other company, the "Swamp Rangers," Company H, was raised by Capt. H. V. Keep, and was mustered into service on August 20, 1861. This company had some Irishmen, but appears to have been recruited among the population at large.

The above two companies had traveled to Memphis shortly after their organization and were casting about for a regiment to join, at the same time that Colonel Marmaduke's 1st Arkansas Battalion was looking for additional companies to bring it up to regimental strength. The battalion was subsequently upgraded to a regiment and designated as the 18th Arkansas Infantry (not to be confused with a later 18th Arkansas Infantry organized in the Spring of 1862), but was redesignated as the 3rd Confederate Infantry in January 1862.

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