Watch Riverdance or another Irish step dance video. Note the way the dancers' legs move rapidly while the upper half of their bodies stay very still. Watch the video in slow motion. The dancers' feet are positioned so the toes of the left foot nearly touch the heel of the right foot, with the right foot turned 90 degrees to the side. Master this frequently used position.
Stand in front of a mirror while you practice. Make your body and leg motions match the leg motions of the dancers. Naturally, you won't get this perfect the first time you try, but if you practice, you will improve. Once you've tried some of the motions on your own, watch Riverdance again. You will notice details that you missed the first time.
Seek advice from the professionals. Learn to Dance offers a step-by-step written tutorial for Irish step dancing. Niall O'Leary's E-how video series on Irish step dance walks you through each part of an Irish step dance routine. Read the tutorial and watch the video, and then try out the moves yourself. This will reinforce your lessons and you'll be more likely to retain what you've learned.
Practice, practice, and practice. The famous dancers of Riverdance did not learn their skills overnight, and neither will you. Practice your Riverdance moves every day for at least 30 days. Keep trying the moves at home, and soon enough you will be ready to reveal your new skills to your friends.