On which instrument was the carol "Silent Night" first played?

Guitar - The Church Organ wasn't working.

180 years ago the carol "Stille Nacht! Heilige Nacht" was heard for the first time in a village church in Oberndorf, Austria. The congregation at that Midnight Mass in St. Nicholas Church listened as the voices of the assistant pastor, Fr. Joseph Mohr, and the choir director, Franz Xaver Gruber, rang through the church to the accompaniment of Fr. Mohr's guitar. On each of the six verses, the choir repeated the last two lines in four-part harmony.

Yay, Fosse4! We always tell the story that it was a mouse which had nibbled through the bellows so the organ didn't work. I'm not sure if that's true or merely a bit of poetic licence, but it was definitely a guitar.