The unarmed 17-year-old African-American was shot to death by Florida neighbourhood watchman George Zimmerman in late February. There have been allegations that race played a role in the killing, but Zimmerman has claimed he was simply defending himself.
Zimmerman has thus far not been charged in relation to Martin's death.
Diddy, Spike Lee, Reverend Al Sharpton and Will Ferrell have all called for Zimmerman's arrest in the past few weeks and now O'Connor has weighed in with a post on her website.
She wrote: "I would like to extend my very deepest sympathies to the family and other loved ones of murdered teenager [Trayvon] Martin.
"I am very sad today (and am certain the whole of Ireland is) to learn of poor [Trayvon's] terrifying ordeal and horrified by the fact his known and named and admitted killer has not been arrested, despite the crime having taken place a month ago. This is a disgrace to the entire human race."
O'Connor also called on the American public to use this tragic story as an opportunity to unite against racial injustice.
"All Americans should deplore this crime. As should all people of all nations. And deplore the fact this man has not been arrested," she insisted. "All Irish people should do the same.
"And I ask that we here in Ireland should express through our American embassy that we would like to see this man arrested this very minute. Because racism is not acceptable. Nor is vigilantism. And this was very clearly in no way at all a case of self-defence."
President Barack Obama expressed his own sympathies to Martin's family on Friday (March 23) at a press conference, where he admitted feeling a personal connection to the case.
"If I had a son, he'd look like Trayvon," Obama told reporters.
Martin was visiting his father in Sanford, Florida at the time of the shooting.
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Watch Reverend Al Sharpton speak at last week's Trayvon Martin rally below: