The best way to learn Irish step dancing is to take a class. Many dance studios offer lessons and classes, and there are hundreds of studios devoted exclusively to Irish dance. You can look for studios and classes organized by U.S. states and Canadian provinces at feisworx.com.
If you aren't able to join a class to learn Irish step dancing, you can also learn the basic steps through video instruction online. Learntodance.com offers a series of videos illustrating the three basic steps of Irish step dancing: Jump Threes, Hop Threes and Sevens. Text describes each step as well, so that you can read about each step as you learn it. DanceJam.com also has a number of instructional videos on Irish step dancing that teach basic and more advanced steps.
You can also use DVDs to learn on your own. Colin Dunne, a star of Riverdance, has a series of Irish step dancing videos called "Celtic Feet." The series "Irish Set Dancing Made Easy" introduces beginners to the basics and then leads them through advanced figures (patterns) and reels. Jean Butler's "Irish Dance Masterclass Video" is another popular video teaching Irish step dance. All of these are available through amazon.com as well as specialty video vendors.