From my experience the irritability is caused by several different things. Now, I'm basing this solely on my 30 years of personal experience in trying to find the answers as to what makes ADD what it is. Also, I am not a doctor or certified professional so this is just from my experience and what I have read. With that said, the following may be useful.
There are several factors which may be contributing to the irritability. First, most people get irritable when their blood sugar is low. ADD people appear to be more susceptible to this type of irritability. Most ADD people appear to have muscle fatigue. If you read the history of a lot of famous ADD people you may notice that a lot of them are tired all the time. Statements such as "he was the laziest boy I ever new" are indicative of this. This muscle fatigue, or chronic tiredness, combined with a lack of appetite in many cases (We just don't get hungry when we should) results in the ADD person not eating when their blood sugar drops too low. Many ADD people appear to only get hungry when they are well past the normal point that most people get hungry if that makes sense. When we do get hungry are grouchy and tired so we go for the easy food like donuts, ice cream, cereal. Etc. which provides a short boost, but then lowers are blood sugar even more once the insulin kicks in. Of course this cycle may not apply to your daughter, but just please realize that she may not be hungry when her blood sugar levels drop and may not want to eat. Possible solutions are between meal snacks or supplements which provide the fuel the brain needs to run between meals such as evening primrose oil, cod liver oil, omega 3 fish oil pills, flaxseed oil, borage oil for example. These are examples of some of the supplements which give the brain the basic elements and energy needed to run thus eliminating the grouchiness. Another way to provide more of the essential oils needed to run the brain is to eat a super antioxidant diet. The reason the Antioxidant diet appear to work is: First, they prevent the essential oils needed to run the brain from oxidizing so the brain has energy and the building blocks of cognition. Second, the natural food used in most of these diets are automatically eliminating a lot of the things which cause grouchiness in ADD people such as additive, sugar, colorings, etc. (Be advised that eating a lot of foods high in antioxidants appears to increase a persons susceptibility to rashes and sensitivity to things like laundry soap and bath soap, etc.) The second contributing factor in irritability appears to be that many ADD people do not absorb their vitamins very well and need supplements. I found the Spring Valley SUPER B-Complex from Wal-Mart works good. I take up to 9 pills a day. Of course check with your doctor before changing your diet or taking any supplements. Their B-Complex sublingual liquid vitamins are also really good from my experience. These are the two key things. There are many more. If you would like more information you may email me at GrouchinessAnswers@thisistheplace.info
Sincerely,
David Lee AKA LightNKnowledge