Irrigation led to the existence of artisans and other specialists because it allowed for a greater amount of agricultural production.  In other words, it allowed people to produce more crops using a given amount of land.

Irrigation allows for more intensive farming.  When crops have ample water, they grow faster and are more productive.  This means that a given area of land can typically produce a larger amount of crops when it is irrigated than when it is not.  Therefore, more irrigation means more food.

The existence of more food means that there can be more people who do not need to farm.  When the same amount of land produces more food, then the same amount of land can support more people.  Some of those people will not need to be farming.  If more people can be supported by the same amount of farming, there are now going to be “extra” people, those who are not needed to farm.  Some of those people can become artisans or priests or teachers or merchants or any number of other kinds of specialists.  Before irrigation, more people had to farm and so there were fewer people who could be artisans or specialists.  After irrigation, there was more food to feed the “extra” people who could become artisans or specialists.