There was no Irish regiment in Cobb's brigade. Several of the companies of the 24th Georgia included Irish immigrants. The Colonel of the 24th GA was Robert McMillan, an Irish immigrant, he and his son were Irish. Company K the McMillan Guards of Habersham County was named for him. McMillan organized the regiment (see thread linked below.)
After Thomas RR Cobb was mortally wounded at Fredericksburg, it was Colonel Robert McMillan of the 24th Georgia who assumed
immediate command of Cobb's Brigade. Shortly afterwards, the 18th Georgia was transferred to the brigade and William Tatum Wofford (formerly Col of the 18th GA) was promoted to command the brigade. It was thereafter known as Wofford's Brigade.
Here are some past threads about Colonel McMillan
https://civilwartalk.com/threads/ir...forms-a-georgia-regiment.122360/#post-1290188
https://civilwartalk.com/threads/ir...h-ga-at-chancellorsville.121156/#post-1268433
https://civilwartalk.com/threads/li...rigade-at-fredericksburg.129020/#post-1428480
https://civilwartalk.com/threads/re...an-24th-georgia-infantry.110141/#post-1059453