The 39-year-old broadcaster is on cloud nine after O'Farrell popped the question and she took to Twitter this evening to announce the happy news.
The Galwegian wrote: "We're engaged!!!
"Overjoyed & overwhelmed.
"I never saw it coming.
"It's still sinking in... I am so happy."
She shared a picture of the couple and their eight-month-old son Cathal admiring the unique ring.
She also revealed that the stunning ring was designed by her sister Grainne - who is behind custom diamond jewellery business 'Grace Diamonds'.
Sile wrote: "Himself & the sister had been in cahoots about the ring for some time apparently!
"@grainne_seoige designed it especially which is incredibly special & I absolutely adore it!"