The Phone Booth star decided to quit his habit on his 34th birthday last year, but before he puffed on his last cigarette, Farrell spent an entire day consuming as many smokes as possible.
And as soon as he finished his last one, the star wrote a special letter to mark the occasion.
He tells Britain's Shortlist magazine, "I gave up the Sunday before I turned 34. I spent the whole day with a packet of cigarettes. I didn't really see anyone and with every cigarette I smoked, I smoked with as much awareness as I could. And then I wrote a little letter to tobacco.
"It said the usual, 'I remember the first time we met and all that we've been through together. That time you helped me through such-and-such a situation...' and yadda-yadda. It was very much the kind of letter you'd write to a person you were breaking up with. Because that's what it was - a break-up. It was the first time I'd ever broken up via letter, so it was strange."