The geek in me was curious and tried to look up their casualty rates. I found as high as ~40% during Gettysburgh and 38% for Fredricksburg (specifically Marye's Heights).
Numbers in the link below have the Irish Brigade's losses at 12%.
http://opinionator.blogs.nytimes.com/2012/12/11/the-fighting-irish-brigade/?_r=0
This one says the 69th suffered 17.1% casualties ranking it 7th among Union regiments.
Fox's Regimental Losses, Chapter 1
A brief rundown of the Brigade's casualties:
The Irish Brigade
Confederate regimental casualties were much higher.
Given the losses of similar non-Irish Union brigades (+80% in some battles) I'd think any talk of them used as cannon-fodder (not here, but I've seen it elsewhere) is more propaganda than fact. Horrific losses to be sure, but sadly not unusual for that war.