I'm a little convinced that 'irritable uterus' means that they don't really know why something's happening...
I was told I have an irritable uterus on Monday, I'm 37 weeks on Friday. My stomach is rock hard/contracting if I stand or sit up, change positions, cough or sneeze, and contracting irregularly when I'm lying down, but not with as much intensity. If I lie or sit reclined, I usually get a bit of a break from it but it starts again as soon as I have to move around. When it contracts, it stays hard for up to 45 minutes, stops for maybe 10, and then starts again. I'm getting back ache with the tightness.
My understanding is that the contractions are preparing your body for labour and I'm hoping it also means that true labour will start very soon, but I've only got hope.
I just found this website http://www.irritable-uterus.com which you might find interesting too.