Dublin actor Emmett, 35 - most recently seen in Jamie Dornan BBC drama The Fall - announced the exciting news on Twitter over Christmas.
The true romantic told fans on Christmas Eve: "There are over a million ways to ask the question, but only one real answer... #WorldsLongestHangoverStartsToday."
He also shared a picture of the couple toasting their good news and sharing a kiss.
Fans wrote: "Huge congratulations to you both, you beautiful couple xxx."
And another said: "Fabulous choice in the ring! It's gorgeous! Xx."
Former Hollyoaks stunner Claire, 34, simply announced the news with the message: "I am a very happy Cooper today... Soon to be Scanlan! Thank you for your lovely messages."
The pair began dating three years ago during their time on the C4 soap. Claire played Jacqui McQueen for six years, while Emmett terrorised as killer Brendan Brady.
Since leaving Hollyoaks in 2013, Emmett has appeared as a cop opposite Gillian Anderson in The Fall, and starred in Hollywood blockbuster Guardians Of The Galaxy.
The couple first went public with their relationship in May 2011 after being spotted kissing on holiday in Mallorca.
Emmett has a daughter Kayla from a previous relationship.
Aside from acting, Claire also co-owns clothing rental boutique The Closet with Hollyoaks co-stars Jennifer Metcalfe and Leah Hackett.
I am a very happy Cooper today ... Soon to be Scanlan! Thank you for your lovely messages— CLAIRE COOPER (@CLAIREECOOPER) December 24, 2014
A huge congratulation to the newly-engaged couple!
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