Especially the ones with Wolfe Tone and David O'Connell. Wouldn't the new US government, especially with Adams at the helm, want to spite the British in any way possible?
It probably goes deeper than that, I know, but what happened?
I'm not an expert in US foreign policy but my understanding is that Adams's administration was trying not to go to war with either Britain or France and was under huge pressure from both. Helping the United Irishmen in 1798 would have been effectively a declaration of war with Britain. There were pro Irish groups in the US at the time.
O'Connell wasn't a rebel, he was a peaceful constitutional reformer. He did receive some help from US supporters but his opposition to Slavery may have reduced this.
Sources
Gordon S Wood, Empire of Liberty George C Herring, From Colony to Superpower
I'm sure you mean Daniel O'Connell.