And the product comes from the kitceh cupboard.
The actress admits she got one of her best beauty tips from her grandfather. Julia is well known for her gorgeous complexion and dazzling white teeth.
The stunning star says she uses items found in the kitchen cupboards to polish her pearly whites rather than spend lots of money on expensive treatments.
"I brush [my teeth] with baking soda," Julia admitted to In Style magazine's September issue. "[My grandfather] would put a big heaping mound of it on his toothbrush. He had only one cavity in his entire life. Sometimes that's all I do as far as a regime goes."
The busy mother-of-three admits she saves time by putting the bare minimum of time into her beauty routine.
Julia says she has some other tricks to looking glamorous when she does not have time to spend on herself. The brunette beauty says big black shades are a girl's best friend when it comes to hiding a multitude of sins.
"Look, I'm happy if I have on moisturiser and ChapStick! When you're dressing three people besides yourself, something has to fall away," Julia continued.
"I've learned to always have a big pair of dark sunglasses within arm's reach before going out."
Julia was snapped this week on holidays with her family in Co Mayo.