Fallon headed in the opener after four minutes and struck a second after 41 after Michael Higdon had missed the chance to equalise from the penalty spot in the last-eight contest at Fir Park.
Motherwell, last season's beaten finalists, had captain Keith Lasley shown a straight red card for a foul on Fraser Fyvie before Nicky Law pulled a goal back but it was too late as the Dons advanced to the last four at Hampden.
Source: PA
Source: PA