The Green and the Grey usefully combines what was already known about the Irish in the Confederate states with original scholarship and numerous corrections and debunkings of received opinion, to present a measured take on a somewhat unwieldy and too often sentimentalized topic. The numbers of Irish-born in the rebel states, and of those who fought for the Confederacy, are carefully examined while at the same time Gleeson convincingly explains the difficulty of arriving at definitive tallies. The Irish presence – largely Catholic and working-class – has tended to be excoriated for lack of dedication to the cause and at the same time commended for the rash bravery of individual participants. Concluding that the Irish were indeed reluctant rebels, never quite as committed as were many of their fellow southern citizens, Gleeson clearly demonstrates the reasons for this reluctance, its wavering trajectories (the defense of their homes became a serious motivation once the fighting began), and its varying reinterpretations of its own loyalties. He acknowledges that Irish troops mutinied at times (especially in New Orleans, though he gives interesting domestic reasons for this), that when captured they were often quite ready to take the oath of allegiance to the Union, and that they were also less hostile to the northern invasion toward war's end; throughout, many Irish conscripts claimed foreign (that is, British) citizenship and sought – and found – protection from the British consulate in doing so. Where the Irish were not different was in their hostility towards slaves – even though only the relatively wealthy among them possessed such – and their conviction that they were of a superior race quite apart from the obvious competition with African Americans for menial labor.

For the Irish, their own sense of nationalism and English oppression fed into their identification with the southern cause and its rebellion against what was perceived as northern aggression; it was a trope drawn upon by many of the secessionist politicians in their efforts to recruit them to the cause, and one occasionally invoked by Jefferson Davis and others to describe the overall condition of the South itself. Their fighting units were named after iconic Irish rebels – Patrick Sarsfield and Robert Emmet especially – and their enemies were identified as the spawn of the hated Oliver Cromwell. Because the Republican Know-Nothings had been so hostile to Irish (and German) Catholics, and Democratic southern leaders notably less so, Irish sympathies tended to favor the latter even in the northern states; in other words, they were often as reluctant to fight for the Union as they were to defend the Confederacy. Still, in his many mini-biographies throughout the text, Gleeson deftly surveys the lives of noted heroes – Patrick Cleburne, John Mitchel, John Bannon – and others less famous.

An especially interesting aspect of this study is its focus on the role of the Catholic Church in the South. In light of subsequent American history, it is surprising to find that it was more tolerated there than in the North (where the Abolitionists were among its leading critics). The Church was the greatest cohesive force for the Catholic Irish and, if anything, its prelates and clergy were even more supportive of the Confederacy than were the ordinary faithful. The Vatican was reluctant to condemn slavery, seeing it as biblically authorized, while Bishop Patrick Lynch of Charleston, a slave-owner himself, was its defender both practically and theoretically. This view was pervasive, though Gleeson presents the case of a Father Peter Whelan of Savannah, who went from being the most ardent of secessionists to, following what he saw at the Andersonville prison, its chastened rejector. Ironically, it was with the fall of the Confederacy that the Irish became most Confederate (Bishop Lynch seems to have altered his personal narrative several times as the forces of Confederacy, Union, and Redemption, respectively, held sway) and, in a way, the defeated Confederacy most Catholic: the postwar mania for commemorations and “feast” days (Robert E. Lee and Stonewall Jackson) was akin to Irish Catholic practice “far more so than [to] most southerners, who had been raised in a more sober and plain evangelical tradition.”

Overall, however, as Gleeson observes, the Irish “had a better definition of themselves than the natives they lived among,” so that “Ultimately, the Irish Confederate story tells us of the weak nature of Confederate, and indeed American, nationalism in the mid-nineteenth century.” Meanwhile, the rhetoric of nationalism, Irish and southern, so often quoted here, now rings so narrow and self-righteous that one eventually longs for the more truly liberating voices of a Frederick Douglass or a Daniel O'Connell.