I do not find a Cassidy on the roster of Lucas' Battalion in Fold3. Where did you find reference to him?

15th SC Heavy Artillery Battalion
Organized: on 6/6/61
Mustered Out: 4/26/65

James Jonathan Lucas, Major, Commanding Battalion of three companies, A, B, & C

Record of Events, Company A for the period July & August, 1863 stationed at Fort Pemberton, S. C. at the end of this period.

The Company embarked for Morris Island on 19 July, 1863, and returning to this post 24 July. Embarked again for Morris Island on 10 August and returned to this Post on 17 Aug. While on the Island was stationed in Battery Wagner and engaged the enemy's fleet and land batteries day and night without cessation.
Capt. Jno H Gary died Aug 17 of wounds received Aug 12. Capt. E. B. Colhoun assumed Command Aug 17, 1863. 1st Lt. G. Richardson transferred Aug 17, 1863.
1st Sergt S. A. Tynes died July 24, 1863 of wounds received July 20.

M267: Compiled Service Records of Confederate Soldiers Who Served in Organizations from the State of South Carolina