"It was a real Louisiana swamp," Ruth says. "Sewage in there, and alligators."
However, she's starring alongside Brad Pitt and Michael Fassbender -- and that would make any woman feel better.
Not surprisingly, Limerick native Ruth said she found the experience a little overwhelming.
"The stuntman told me not to worry, it was only knee deep. Knee deep for him!"
The petite actress, who is just five foot two inches, sank up to her armpits in the water. But Ruth says she would do it all again if director Steve McQueen asked her to.
"There's absolutely no b******t about that man whatsoever. I went into a swamp for him. I think he's great."
The film is based on Solomon Northup's 1853 autobiography of a man who was born free and forced into slavery.