I would think aliens are far more likely to occur in the universe than leprechauns. I would say its a certainty there is other life, whereas leprechauns are certainly non-existent.

The claim to be falsified isn't that aliens exist anywhere in the universe, which is likely, but specifically that they are on Earth, and by extension, that stories of UFO phenomena (sightings, abductions, etc.) are accurate and correct.

Conflating the two is a common misdirection tactic of UFO believers, meant to imply that it's as reasonable to believe in the former as the latter, or that the general scientific consensus that alien life probably exists somewhere in the universe lends credence to UFO mythology.

The truth is, without some evidence that everything we know about physics is wrong, the universe could be teeming with life and it would still be about as reasonable to believe in the presence of extraterrestrial life on Earth as to believe in leprechauns.