odudog

If you were to head butt uke with your forehead, would you hit uke's face or back of the head. If you hit the face, then you are in omote and if you hit the back of his head, then you are in ura.

Irimi just means how you will move your feet so that your entire body will move. If one foot moves in front of the other {like you are walking} and you are still looking in the same direction then you are doing irimi. If one foot moves in front of the other, but then the other foot sweeps behind so that you are now looking in the opposite direction then you are now doing tenkan. This is just a simple explination to get you started in the right direction but is not a hard and fast rule as you will learn as continue to progress in the art. Jun's explination where spot on explaining the differences within irimi, tenkan, omote and ura.