Jack,

Dekalb County was earlier populated by English, Scottish & Irish immigrants.
I could not find anything that mention either the "Murphy Guards" or " McCullough
Rifles" as being Irish-units. There were some in the "Murphy Guards"; but probably not exclusively.

"DeKalb Murphy Guards"/ "Murphy Guards"
"McCullogh Rifles"

"Camp Kirkpatrick" DeKalb county(near Decatur)

Some of my ancestors were members of a Irish company formed in Atlanta; but the members were not entirely Irish {composed of a number if Irish-immigrants or men of Irish-descent. The Officers/NCO's were Irish and some of the men, but not all