According to Britain's The Sun, they enjoyed a trip to posh hotel Babington House in Somerset, England over the weekend (23-24Jul11), where Carey Mulligan showed off her sparkler.
A source tells the publication, "Carey looked like the cat who had got the cream. She was wearing a beautiful ring and didn't seem to mind who saw it.
"They were stuck to each other like glue. She was doing most of the talking and he was hanging on her every word."
Carey Mulligan previously dated her Wall Street: Money Never Sleeps co-star Shia LaBeouf, while Marcus Mumford was in a relationship with fellow folk singer Laura Marling.