While there was sympathy for the Irish, it was not the most pressing at the time as England was involved in WWI with Germany trying to get the U.S. involved (however, the U.S. had desperately tried to stay neutral, but ties to Britain, propaganda, and finally the sinking of The Lusitania finally got them involved in 1917).
The Rebels soliciting arms and ammunition from the Germans, Roger Casement’s failed attempts to raise an Irish Brigade from POWs held in Germany certainly did not help their cause. The men of 1916 also acted independently, they were instructed by Eoin MacNeill, prior to the Rising to stay at home as Ireland was (supposedly) within months of getting Home Rule.
There were a couple of U.S. fact-finding missions to Ireland post-1916, but they were not there to provide support for the rebels, but monitor the treatment of civilians by the RIC and British Forces. Their visits were seen as interference and barely tolerated when visiting locations throughout Ireland.
Image | A group of women on the sidewalk, carrying signs against America’s involvement on the side of the United Kingdom against Irish Republicans, and tearing up a British flag, June 3, 1920 | Wikimedia Commons, Library of Congress