There is nothing wrong with dialogue--it just slows down reading.
The picture is an anonymous painting of the African American troops at the scene of the battle. The book is an excellent fictionalized telling about this episode in the history the USCTs, but not such a good one about the history of the Irish.
Remember Buster Kilrain, from Gettysburg? Sure and did ya ever think ya'd hear (or read) another mick with a fondness fer the bottle callin' his commanding officer "Colonel darlin'?" Well, he's in The BoC.
And remember the grumpy Irish drill sergeant in Glory who made life hell for Andre Braugher? Well, faith and begorra if he didn't NOT die at Fort Wagner, but lived and came to drill the men of the 28th USCT. Or maybe all Irish drill sergeants are like that guy--or not.
Anyway, for a book that tries to set the record straight about the USCTs, it does one heck of a lousy job with Irish stereotypes.