The Phone Booth star was a surprise mourner at the Cleopatra icon's funeral in 2011, and he later revealed they had been friends for 18 months before her death that March and he has now told how they got to know each other after a chance encounter.
Farrell was at a hospital in October, 2009 for the birth of his son Henry when he heard the actress was undergoing the operation, and he later arranged to send flowers to Taylor but was stunned to learn she had already sent him a bouquet.
They struck up a friendship, and the actor reveals he often used to call his pal in the middle of the night for chats.
He tells the Daily Mail: "I was at the hospital when Henry was being born, and about an hour or two before he came into the world, I went for a smoke, and a couple of guys came in and said they were with Elizabeth Taylor, and I said: 'Oh, is she all right?' They said: 'She's fine, she's just having a stent put in her heart, it's no big deal.' I said: 'Tell her I said hello, although she probably won't know who I am,' and they said: 'Oh, she knows who you are all right' - which was one of the most pleasing things I'd ever heard in my life.
"So Henry was born, we got home, and two days later I called my publicist and said: 'Can you send some flowers to Elizabeth Taylor from me, please?' She said: 'That's funny, but there are some flowers here from Elizabeth Taylor to you!' And then ensued the most unique friendship I've ever had, which lasted for the last 18 months of Elizabeth's life.
"I used to call her at 3am because I sleep at crazy hours and she was an erratic sleeper, too. She was a great friend and a faithful friend, and, oh, I loved her."
The Hollywood legend died of congestive heart failure on 23 March.