Irritability is common and has many causes. Everyday frustration at the loss of memory and thinking ability is part of it. So too is the loss of ability to modify behaviour - to 'keep a lid on it', which comes about as part of the disease itself.
This symptom can be distressing for families, especially when it is out of character for the person. If you can, it often helps to step back from your immediate, emotional reaction - as a carer once said to me, "to see the person, not the disease". But remember that this is one of the hardest parts of the disease for caregivers.
Improvement in irritability can be seen with drug treatment . However, sometimes the pattern of irritability, and the way in which it recovers are not well captured by the standard ways we test drugs, which is why we are targeting it here- to give people a voice in how the drugs work. This issue is described in more detail in Rockwood K et al., Potential treatment effects of donepezil not detected in Alzheimer's disease clinical trials : a physician survey. Int J Geriatr Psychiatry. 2004 Oct.19(10):954-60.