Brianna, who was a transgender teenager from Birchwood in Warrington, was found by members of the public as she lay with fatal stab wounds on a path in Culcheth Linear Park on Saturday.
On Wednesday evening people gathered in Dublin at The Spire on O’Connell Street, while in Belfast they came together at the front of the City Hall.
Protesters carried placards reading “Rest in Power” and “Trans Rights Now”.
Two teenagers charged with her murder were remanded into custody by a district judge on Wednesday.
In Dublin, many of those who attended the vigil lit candles and left them on the pavement in front of a portrait of Brianna.
Academic and LGBTQ activist Ailbhe Smyth described the teenager as “strong and fearless”.
She said: “She was only 16. She was on the cusp between girlhood and womanhood, that very delicate and also very wonderful moment.
“I felt her family’s grief so much when they said in their beautiful statement about the death of their daughter.
“They said she was strong, fearless and one of a kind. I thought of that young woman and how brave she had been in her life to say I am a trans woman.”
Solidarity councillor Leah Whelan told the vigil: “Brianna was only 16.
“She deserves recognition, she deserves respect and she deserves to be alive today.”
In Belfast, many of those who attended the vigil carried photographs of Brianna.
Leo Lardie, from the Rainbow Project, told the vigil: “Usually I am known as a man who is never at a loss for words, but today I truly and utterly am.
“There are no words to describe how hopelessly tragic and keenly painful the senseless death of a young girl like Brianna is.
“We are here tonight to show our deepest sympathies for her family, friends and the trans and non-binary community of the UK all of whom are left bereft by her sudden death.
“The UK can feel like an increasingly frightening place for trans and non-binary people to exist.”
Alliance representative on Belfast City Council Micky Murray said the death of Brianna was being felt by the trans community right across the world.
He said: “I hope this event brings comfort but I also think it brings a level of discomfort.
“Obviously we don’t want to have events like this, we don’t want to have to stand here to protest and to hold vigils.”