The Express reports 33-year-old Dockery, who plays Lady Mary in the period drama, was seen wearing an engagement ring at a recent script read-through.
"She looked over the moon and was showing it off to all the cast and crew," a source told the newspaper. "It's wonderful news and everyone is delighted. She and John make a lovely couple."
Dockery has been dating Dineen, who works in financial PR, since 2013 after she was introduced to him by her Downton co-star Allen Leech (he plays Tom Branson).
There are no details on when the couple are planning to get hitched, but it sounds like Michelle is keen to make a style statement when she does walk down the aisle.
"I enjoy dressing up much more than I used to," she told M&S Style and Living magazine in December.
"I think I went through a phase when I thought that only a certain style suited me. But I learnt that as you get to know fashion a bit you can mix it up. I've always said my style is classic and simple, but lately, I have started to play around a lot more with colour and style."
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