As pointed out, there was no such thing as Northern Ireland and the UI had every intention of creating a united Irish republic.
America is a million miles away. The Atlantic passage took long enough at this point that by the time news of a French effort to assist the UI got to America, they'd probably have won or lost already. Owing to Britain's naval supremacy, it would be a campaign decided early on. Further, of course, America had no desire and little ability to project land military power (or any military power) beyond its most immediate neighbourhood. Nor did it have any reason to sympathise with the Irish for themselves, this being at a time when "Irish Americans" were almost entirely Presbyterian Ulstermen, and were a much less significant demographic than later anyway. There were those in America who sympathised with France and with Britain, and those are the only interests that matter here.
Basically, American involvement is one big red herring based on back-dating the rather green-tinged view Americans tend to have of the island's history.
French involvement is a lot more interesting. In the age of sail-ships and the mark-one eyeball, there is simply no way infalliable way to prevent the enemy getting from A to B; indeed, the RN failed to do much about the French expedition in 1796, which was foiled by command confusion and one of the vilest winters on record.
The thing to do is to make the French invasion come earlier. The UI were actually weaker in 1798 than they had been earlier, having been ruthlessly repressed. If the French had been able to land at some earlier point in the 1790s, avoiding rotten weather, the rebels would have had a shot at victory.