As far as the Sullivan fable, it has pretty much been shot down by serious historians. Kelly O'Grady pretty much disected it very well in his book "Clear the Confederate Way". There was No Mike Sullivan Listed in the rosters of the Reb Units in question and the thought that a Lone Reb could cross his lines, plus wander through a mile or so of Union troops and picketts, swim an ice cold river and find a specific Brigade commander in the dark is Fantasy.
Its also been documenetd that the 69th Irish Flags was not present at Ferdericksburg. It was being repaired after being sht up at Sharpsburg, hence the Irish wore sprigs of Boxwood in their caps.
The 69th's Flag was presenedt to the Irish Parliment by JFK during the war's Centenial, and thats where it is today.