The Ericsson seemed to have no trouble getting to Ireland so supply the rebells with guns and go back. Strange that Britain had no patrolls either by air or by sea on the west coast of Ireland.
I wonder if the Ericsson went by way of Iceland. The Danes are neutral in the war, so theoretically a US destroyer could stop off at Reykjavik to get refueled while traveling to and from Ireland. It would be rather risky for the Danes to allow that though as it would definitely upset the British and might lead to Iceland getting a visit from the Royal Marines. (And at least IOTL, the Danes signed a declaration in 1904 declaring their ports would not be open to belligerent warships in time of war.) Still, if the US and Germany apply enough pressure then the Danes might allow it.
Otherwise, sailing from North America to Ireland and back in a single trip appears to be just barely doable for a World War 1 era destroyer.
IOTL the USS Clemson, a Wilkes Class destroyer (one of the most common types built by the US circa WW1) that had been modified to give it 35% additional fuel capacity had an estimated operating range of 4,900 miles.
The Clemson was commissioned in 1919, but it's plausible enough in TL-191 that the pre-FGW US, lacking access to Pearl Harbor, would have focused on building some extra long range destroyers earlier than it did IOTL in anticipation of having to fight way out in the Pacific.
The sailing distance from Halifax to the port of Galway in western Ireland appears to be just a hair over 2,450 miles.
Thus a long range US destroyer from that era sailing from Halifax (after its been captured) could just barely make it to western Ireland and back. It would be a very risky operation though since the destroyer would be running on fumes by the end and if it had to do any evasive maneuvering at all on its voyage then it would run out of fuel on the way back. Going by way of Iceland would be much easier if the Danes will let them refuel there.