There is an occasional increase in sleep problems. Some toddlers get very active. Some toddlers find it very difficult to settle down and pay attention even though they were able to do that wonderfully well a few months earlier. And it's like being propelled into a whole new environment with lots of new skills. If we found ourselves in this situation, I think we would be irritable too.
Some children get irritable if their noses are wiped.
Some get irritable if they're told they have to go outside. Just swoop up the toddler, grin and bear it. When we talk about irritability we have to talk about temper tantrums. And the reason is that at 18 months of age, toddlers don't have a lot of language. That brings a certain amount of frustration.