I don't know about a Guinness organ, but they were giving away mini Guinness fiddles over the St Patrick's weekend. Buy 5 pints to qualify, and the fiddle plays 8 tunes. Either on 'Demo' setting, or one note at a time when the metal string of the bow makes contact with two of the metal 'strings' of the fiddle.
The alternative gift was a large, floppy, Guinness hat for buying five pints.
If an excuse for drinking ten pints of Guinness were required, then this would be it. But, who needs an excuse ??