I kept a food journal for a different reason - I was newly diagnosed with Rheumatoid Arthritis and in constant pain, and my consultant wanted to identify any possible trigger foods. For a month, I wrote down everything that passed my lips, and recorded how I felt the next day. From that, I was able to identify red wine as a trigger, and also red meat, if I ate it for two days running. I now avoid red wine, and only have red meat occasionally.
It's not so time-consuming as you think to just jot down what you eat and drink - there's no need for portion sizes or weighing, or even the times you eat, if you don't want to bother, although with IBS that could be helpful. It's certainly time well spent, so it's worth making the effort.