The 31-year-old Irish singer, who split from Delta Goodrem in April last year, said that he asked 26-year-old DJ and model Williams to marry him while he was touring with Stevie Nicks in November.
After posing for photos showing off the ring, McFadden told the Australian edition of OK magazine: "It's early [to be engaged] when you think about how long we've been together, but not when you consider the amount of time we've spent together… we've literally spent every second of every day together for eight months now."
Admitting that he "never really planned to get married" to Goodrem, McFadden added: "Vogue and I are actually setting a date and we're actually getting married."
After splitting from Goodrem, McFadden said that he planned to be a "hermit", but then met Williams in Ireland while visiting his daughters Molly and Lilly-Sue, whose mother is McFadden's ex-wife Kerry Katona.
The star said that Goodrem and Williams's most recent ex-boyfriend are happy about their engagement news, saying: "We've let them know out of respect. They both went, 'Great, as long as you're happy,' because they're both happy as well and have moved on.
"We don't have any nasty 'ex' animosity with our recent partners."
McFadden and Katona have had a famously turbulent relationship since they separated in 2004. He recently was said to be "disgusted" at her accusations that he "rarely rings" his daughters.
> Vogue Williams moves to Australia for Brian McFadden
> Brian McFadden 'may not return to Australia's Got Talent'