1. 30' to 40' is enough distance to not be a problem at least for a significant number of years. Whether a problem will ever occur may depend on factors like the design of your septic field. Newer installations are hopefully less prone to damage (better designed) than many older systems. You may want to consider damage to the tree from eventual septic system maintenance years down the road.

2. A well-irrigated tree may require a smaller root system, but will also be less well suited for things like strong winds and unusual droughts. I don't really think relying on irrigation to control root growth is a great idea. But, yes, it can be done.