You can order the pension and service records here. This young man (given as age 17, born Ireland, when enlisted) joined a state cavalry company raised by Capt. Silas H. Wood, Pontotoc MS, Feb. 20, 1864. The company belonged to the 2nd Mississippi State Cavalry Battalion, brief unit history given here --
http://mississippiscv.org/MS_Units/2nd_bat_st_cav_Harris.htm

This battalion was turned over to Confederate authority on May 4, 1864, Tupelo MS. Capt. Wood's company became Co. "C", Ashcraft's 11th Mississippi Cavalry Regiment. Here are further unit histories --
http://mississippiscv.org/MS_Units/Ashcrafts_MS_CAV.htm
http://mississippiscv.org/MS_Units/Ashcrafts_MS_CAV.htm

D. T. Long received his parole at Columbus MS as a member of Co. "G", 11th (Ashcraft's) consolidated Mississippi Cavalry Regt.