The Irish-American ballad that an intoxicated Lieutenant Kevin Riley sings over and over through the ship's intercom after locking himself in engineering in The Naked Time.
I'll Take You Home Again, Kathleen is a nineteenth-century Earth ballad. In the first-season episode The Naked Time (2266), the crew of the Enterprise is infected by a waterborne contagion from the dying planet Psi 2000 that strips away inhibition.
Lieutenant Kevin Riley, fancying himself the descendant of Irish kings, seizes the engineering section, shuts down the engines and announces that he is now captain. He then treats the whole ship to repeated, tuneless renditions of the song, promising one more time after each.
While he sings, the powerless ship spirals towards the collapsing planet. Scott has to cut through a bulkhead to reach him, and the engines are restarted by an untested cold-start formula that throws the Enterprise three days back in time.