I figure the two Regular brigades together had more Irish-born soldiers, although admittedly it is based on a rather unscientific method. In the following calcuations I assume the casualties randomly represent the unit as a whole in terms of birth country:
In the two Regular brigades, I list 50 casualties whose birthplace was Ireland. Busey and Martin's Regimental Strengths shows 829 casualties out of a total engaged strength of 2,507 in the two brigades. Therefore the Irish-born comprise at least 6.03 percent (50/829) of the casualties, which extrapolating, indicates the two brigades together had at least 151 Irish-born men (.0603 X 2,507) in them.
As for the Irish brigade, I list 33 casualties whose birthplace was Ireland. Busey and Martin's Regimental Strengths shows 198 casualties out of a total engaged strength of 532 in that brigade. Therefore the Irish-born comprise at least 16.67 percent (33/198) of the casualties, which extrapolating, indicates the Irish brigade had at least 88 Irish-born men (.1667 X 532).
Of course, here we are comparing two brigades against one, which is not quite fair, right?