Conor McGregor Mourns Sinéad O'Connor's Death, 'An Irish Treasure'

Conor McGregor was devastated to learn of the passing of musical icon Sinéad O'Connor on Wednesday ... telling TMZ Sports he considered her "an international star and an Irish treasure."

The UFC fighter grew close to the singer throughout his career in the Octagon ... particularly after she sang during his walkout at UFC 189 in Las Vegas.

You'll recall ... during the July 2015 performance, she belted out "The Foggy Dew" -- and many have since considered it one of the best entrances to a UFC fight ever.

In fact, McGregor himself called it "an honor to go out to that" ... adding that it was "go-to-war music."

Following her death this week, McGregor told us in a statement he was absolutely heartbroken.

"The world has lost a beautiful voice and an even more beautiful person," he said. "I am honored to have known her, to have played her music both in and out of the Octagon. God rest Sinéad."

He later added in a tweet, "Ireland has lost an iconic voice and one of our absolute finest, by a long shot."

The Irish Times was first to report the news of O'Connor's passing ... though the circumstances surrounding her death are still unclear.

She was just 56 years old.