A recent randomized, single blind study published in the Journal of Endodontics supports this practice as a way achieve more effective canal sterilization. Carver, Nusstein, Reader & Beck showed that canals cleaned and irrigated normally, followed by a one minute ultrasonic irrigation with an ultrasonic needle in a MiniEndo unit resulted in statistically significant (p = .0006) reduction in CFU count and positive cultures (p = .0047). Logistic regression showed that ultrasonic irrigation was seven times more likely to yield a negative culture. Source: Carver, K., Nusstein, J., Reader, A., Beck, M. In Vivo Antibacterial Eficacy of Ultrasound after Hand and Rotary Instrumentation in Human Mandibular Molars. Journal of Endodontics 2007;33:9:1038-1043.
There are many ways to perform ultrasonic irrigation. I simply irrigate the canals normally using a disposable syringe and 5.25% NaOCl. Then I use a small stirring tip attached to my ultrasonic handpiece. I am certain there are other products on the market, but I like to use the instruments that I already have.