The Fenian Brotherhood favored the Irish or Fenian Sunburst Flag as its emblem in the Civil War

In the mid-19th century the Sunburst motif was widely adopted by Irish Republicans in Ireland and beyond as a symbol of their movement and of their revolutionary ideology. 

Fenian flags tended to conform to the green of Irish nationalist banners rather than the traditional blue and usually showed a pictorial golden sunburst emerging from behind a cloud instead of from below the horizon (with or without the traditional gold Harp). Most Sunbursts were centred in the middle of the flag but some placed the image at the bottom or top-right.

During the American Civil War (1861–1865) many Irish-American regiments and units made use of the Sunburst in their flags (invariably combined with the more conventional Harp). Those that did so normally had large numbers of Fenians in their ranks or were based on previously existing Fenian organisations or groupings (this phenomenon was largely confined to the armies of the Union: for reasons of geography and ideology Confederate forces tended to attract far fewer Fenians from the Irish-American communities).

http://ansionnachfionn.com/seanchas-mythology/an-gal-greine/

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Charles Goddard and Matthew Marvin visit the Irish Brigade on

Saint Patrick’s Day March 17, 1863

Charles Goddard of Company K, 1^st Minnesota Volunteer Infantry attended the festive St. Patrick’s Day celebration on March 17, 1863 at the headquarters of the Irish Brigade near Falmouth, Virginia.  Young Goddard, 18 was drawn to danger when he was off duty.  Charles Ely, his comrade in arms, said of Goddard, “the only thing he did better than getting into scrapes, was getting out of them.” Matthew Marvin’s diary entry for March 17 indicates he also attended this event. His view of the celebration offers a contrast to Goddard’s.

Notices of the upcoming celebration including a Military Mass, a horse race, and other festivities, no doubt captured many soldier’s interest including Goddard and Marvin. The crowd at the brigade’s headquarters in Falmouth was estimated at over 20,000 people including General Joseph Hooker, Commander of the Army of the Potomac. Under the flamboyant leadership of Brigadier General Thomas Francis Meagher, the Irish Brigade was one of the most colorful units of the Army of the Potomac. The Irish Brigade reflected the characteristics of its leader; it was ferocious in battle and rambunctious between campaigns. 

Matthew Marvin’s diary account of the event.

Tuesday March 17

St. Patrick’s day is a big one in the Army of the Potomoc
Horse-racing and steeple chases is the program lots of
whiskie & lots of fun most all the Gen in this part
of the army are present here  Heavy firing up the river
Weather pleasent mud knee deep.

 http://www.winonahistory.org/companyK/st__patrick’s_day__1863.htm

http://www.hauntedfieldmusic.com/DK.html

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