Reclaiming Irish Citizenship
One interesting note - while it is possible for Irish-born citizens to reclaim citizenship after renunciation, those who acquire Irish citizenship through inheritance (parent, grandparent, marriage) cannot reclaim citizenship if it is renounced.
There is plenty of case law regarding the surrender of dual citizenship for purposes of working for the U.S. government. One particularly interesting case centering on a woman who obtained Irish citizenship through a grandparent, who did not want to surrender it to continue with her job working for a defense contractor, was settled in her favor when an administrative judge ruled that the woman never did anything to actually behave as an Irish citizen, such as obtain an Irish passport. (The case can be found at www.dod.mil/dodgc/ doha/industrial/00-0559.h1.html).
Another site of interest regarding dual citizenship and how it affects those seeking careers at the U.S. State Department can be found at http://careers.state.gov/ print/specialist/join/dualcitizen.html.