Lizzie developed a crush on Robert Sheehan's character, Darren, after the pair were contracted to kill Elmo and his cousin.
When Caoilfhionn auditioned for the show, writers specifically wrote a part for her.
The Dubliner said she relished exploring such a strong character.
"She is written so well; it such a great help," she told the Herald.
"Because Lizzie is so open as a person, it can be a bit of a challenge to strip back all that you know and the things you carry with you from your own experience in life.
"I approached her as an animal-like child. She is trusting and loyal.
"It was interesting to play that off these other characters
"She is constantly surrounded by strong men, aggressive men and to react off them was interesting."
Love/Hate has been criticised for the way it treats women, with some critics claiming that women are simply used and abused by men.
"I've heard that a lot, but it is too easy to label them. I wouldn't use the word 'abuse'," Caoilfhionn said.
"The show does depict physical abuse -- and in the case of Georgina both mental and physical abuse.
"But it is too easy to slap the label victim on the women in this show.
"Everybody is a victim of the circumstances that people find themselves in.
"But Trish is a strong woman, Mary was a very strong woman," she added.
"Lizzie is different in as much as she is not going out or married to anyone. She is outside the gangland thing and is the sister of two lads in the RIRA.
"I think everybody is well written."
Caoilfhionn, who is originally from Finglas and is now living in Phibsboro, said her character develops in upcoming episodes.
But she won't reveal if anything happens between Lizzie and Darren.
"I think it is a mixture of things -- I think she does fancy him, that is pretty obvious," she said. "There is a weird kind of hero worship going on.
"She looks up to him, she is fascinated by him. He is unlike anyone else and so very different to the other men in her life.
"From the get-go, he is kind of looking out for her but I think he is wary of that fact that she fancies him and he doesn't want her to get any ideas.
"They do have a lovely relationship, a loyal relationship.
"They were both there that night when they had to do that job, so they are bound by that."
Caoilfhionn has just finished recording a radio play, the Vision Service, written by Luke Clancy, for RTE.
And next week she drops the gritty crime genre to start rehearsals for Shakespeare at The Abbey.