The caption reads: “The 69th (Irish) regiment embarking in the “James Adger” for the war, April 23, 1861.” Someone has written “May 11 1861” and “Brooklyn” above the caption. They are being cheered by a sizable crowd and one solider is holding a flag with what looks possibly like the emblem of a harp on it.
If you have an interest in the participation of the Irish in the Civil War then you should consult the excellent and aptly named website, Irish in the American Civil War.
Source: New York Public Library Digital Gallery
Note: Thanks to Harry Smeltzer of the Bull Running's blog for pointing out that this is a picture of the 69th Regiment, New York State Militia and not the 69th Infantry Regiment (New York).