I would tee off of the main line prior to the water tanks for the irrigation. Right after the tee, install a stop-n-waste to be able to shut the irrigation off without disrupting the rest of the water system. After the shut off valve, you must use a backflow preventer to avoid the chance of contaminating you domestic water supply.
An expansion tank is needed if you have a pressure reducing valve (PRV) as that creates a "closed" system. The PRV prevents the expansion from being absorbed by the water main and results in a tripped TP valve on the water heater(s). So unless you install a PRV, there is no need for an expansion tank.