Ball below feet = slice

[simpson]

My home course has a few holes where the ball is often on the fairway but below the level of my feet. This is often at the longer holes and I am using my long irons. I usually hit the ball cleanly but the path of the ball takes an almighty slice from my left to right and into trouble. Have you got any advice to ensure that I hit the ball more consistently straight ahead?

Thank you

Bob