The deputy First Minister joined a crowd of hundreds at Belfast City Hall to remember the murdered teacher.
A vigil in Belfast for the murdered Co Offaly teacher Ashling Murphy has been described as a “watershed moment” in highlighting violence against women.
A small photograph of Ms Murphy was laid on the ground alongside a banner which read: “Her name was Ashling”.
Dozens left bunches of flowers and lit candles, which were placed in front of a photograph of the 23-year-old, who was slain on the banks of the Grand Canal outside Tullamore on Wednesday afternoon.
Young musicians who knew and had played with Ashling Murphy performed, before a minute’s silent followed a poem highlighting the issue of violence against women.
The crowd included Sinn Fein MLA Caral Ni Chuilin and SDLP MLA Matthew O’Toole.
One of the vigil’s organisers, Emma Gallen, said: “We all saw the news, we all saw about Ashling Murphy, and we all thought we had to do something.
“We couldn’t just stand by and not acknowledge that a murder happened in daylight of someone who was at the beginning of her life.
“We wanted to mark the grief that we all feel and to come together to mark how sad we all are, and how angry we all are.”
Ms O’Neill said there had been an “outpouring of grief” following the murder.
She said: “At the start of this week I launched a strategy. I called for views in terms of developing a strategy to tackle male violence against women.
“Little did I know on Monday that I’d be standing here only a short number of days later, joining with other women who have gathered here outside Belfast City Hall to show solidarity to the family and all that loved Ashling Murphy.
“A horrific murder of another woman in our society at the hands of a male.
“We are all here because we want to show that support, that love.
“There has been an outpouring of grief all week for Ashling Murphy and for all that loved her.”
The deputy First Minister said vigils taking place across Ireland showed women demanding violence against them needs to stop now.
She said: “We are here because we are saying, ‘Enough is enough.’
“It needs to stop – the violence against women and girls needs to stop now.
“Male violence against women and girls needs to stop now.
“I think the sheer fact that, right across every town, village and county across this island today people are gathering in large numbers to remember Ashling Murphy, shows that women have had enough.
“We are entitled to feel safe, we are entitled to be safe. We are entitled to go for a run.
“We are entitled to go to work and feel safe, we are entitled to go to the shops and feel safe.
“I think this is a watershed moment in our society today.”
Events took place at a number of other locations in Northern Ireland including Londonderry and Newry.