Are you familiar with the letter written by Columbus Gray to his father and family informing them about the death of his brother "Ad" and his description of the fighting? He and his brother were in Co. D, Pleasants' Regiment. I quote the following from the letter:
"Colonel Pleasants was wounded in the first charge his leg was broke(.) he sot on his horse and give commands until his Horse was shot from under him(.) Colonel Gohagans horse was shot from under him but he was not tutched."
So it certainly appears that Lt. Col. Geoghegan was present during the battle. And since Columbus states that he was not touched, I would imagine he assumed command of the regiment after Col. Pleasants fell.