I did my half marathon in 2010 (and promptly quit running seriously after that) 3 weeks after Bloomsday that year, in training up to that I did my standard 3-4 day/week work out, with the general plan being short-short-long and short-short-medium-long (short = 45 min, medium = 60 min, long = >60 min) and I did decent on my half 2:12:00 which I was thrilled with, but I do think that if i had done 4 days/week consistently I would have had better results and likely wouldn't have totally messed up my knee :)
That being said, the 3day/week plan worked but you need to be more focused than i was, running with a plan so to speak. I used to just run to run...which is probably good in it's own right :)
After that rambling, I doubt i'll ever attempt a half marathon again, my body was just too pissed off for too long afterwards, but i certainly will cheer anyone on that starts the training for it and goes out there and finishes strong!