The 25-year-old American singer and 28-year-old Australian actor went all out while celebrating the holiday with friends Saturday.
Cyrus dressed for the occasion in a green Care Bears onesie and a green and silver hat decorated with shamrocks. She shared several posts on social media from the fête, including a photo of herself making a face for the camera.
"Itszzzzz EZ being green! Happy St Pattys," she captioned the picture on Twitter.
Itszzzzz EZ being green! Happy St Pattys pic.twitter.com/A8PCR0VAq0— Miley Ray Cyrus (@MileyCyrus) March 17, 2018
The "Malibu" singer also shared a video of herself dancing around the house, which she had decorated with green balloons and streamers, a huge rainbow piñata and a pot of fake gold coins.
"Put my mind on it .... then I put my grind on it," she wrote on Instagram Stories.
Hemsworth also dressed up in a white and green soccer outfit with green leggings, socks and a tie. He posted a photo on his Instagram account of himself sitting amid a sea of green decorations.
"Happy St. Patrick's day Ireland! Thanks for letting all of us celebrate it with u!! My grandma was Irish and I couldn't be prouder. Light it up!" the star captioned the picture.
Cyrus and Hemsworth reconciled in January 2016 after ending their engagement in 2013. Entertainment Tonight reported Sunday that the couple have started planning an intimate wedding in secret.
"Miley has come up with all sorts of different wedding plans. She is always creative and thinking of new ideas that will make her experience different," a source said.
"She hasn't announced anything because she is being very secretive so only their friends will know and at the last minute," the insider added. "This time they are doing everything their own way."