From Word-Detective:
I can tell you that "Katie bar the door" is a colloquial expression meaning "look out" or
"get ready for trouble," and that it is heard primarily in the Southern United States. Beyond that, things get very murky. I even
searched the Internet for an answer, but all I discovered about "Katie bar the door" on the Net is that the phrase may or may
not -- opinions vary -- occur in the lyrics to an old REM song.

Eventually, like Dorothy in The Wizard of Oz, I realized that the best
answer I was likely to find was in my own back yard all along. My parents,
in their Morris Dictionary of Word and Phrase Origins, report similar
difficulty in tracing "Katie bar the door." In their case however, a helpful
reader came to the rescue and noted that the phrase most probably came
from an old English folk song. In the song, Katie and her husband are
arguing, and somehow agree that the next one to speak will lose the
argument. Since neither will speak to suggest barring the door at bedtime,
robbers break in during the night and commit various outrages against the
pair. The end of the song apparently involves the husband crying out at last
and repelling the miscreants, thereby losing the argument with his wife.
Since I don't have the actual lyrics to the song, I can only presume that the
phrase "Katie bar the door" occurs as a refrain or concluding stanza, but
the accepted meaning of the phrase certainly fits the story conveyed in the song.