McGregor, who commonly used O'Connor's vocals from her collaboration with The Chieftains on their version of the Irish ballad 'Foggy Dew' and had her perform live at UFC 189, shared a post on social media in which he honored the late singer.
"The world has lost an artist with the voice of an Angel. Ireland has lost an iconic voice and one of our absolute finest, by a long shot. And I have lost a friend. Sinead’s music will live on and continue to inspire! Rest In Peace, Sinead you are home with your son I am sure," McGregor wrote in a post shared on social media, which included a photo of himself with his arm wrapped around O'Connor and another of him pointing up at her as she sang him to the octagon during his win against Chad Mendes at UFC 189.
McGregor also issued a statement to TMZ Sports in which he called O'Connor "an international star and an Irish treasure."
"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."
O'Connor, who rose to global fame with her 1990 cover of Prince's song 'Nothing Compares 2 U,' died at the age of 56 on Wednesday, the Irish Times initially reported. O'Connor's cause of death has not yet been determined.
The Dublin native recorded 10 studio albums during her career, which spanned five decades and was presented with the inaugural award for Classic Irish Album at the RTÉ Choice Music Awards for I Do Not Want What I Haven't Got earlier this year.
O'Connor's death comes months after her 17-year-old son, Shane, was found dead in January. The Irish singer found early success with her debut album The Lion and the Cobra in 1987 and made a global breakthrough with her second album, I Do Not Want What I Haven't Got in 1990, with her rendition of 'Nothing Compares 2 U' reaching No. 1 in several countries and charting for 11 weeks, as well as earning her a Grammy for best alternative music performance, as well as nominations for record of the year and best female vocal rock performance.
O'Connor also famously sparked controversy at the height of her popularity when she held up a photo of Pope John Paul II while reciting the lyric "evil" before ripping it apart while performing an a capella version of Bob Marley's 'War' on Saturday Night Live in 1992, concluding the performance by exclaiming, "fight the real enemy," in protest against sexual abuse of children in the Catholic Church.