I would think you would need some sort of holding tank and a pump to bring it back up to pressure for the irrigation system. The tank will need a overflow outlet for the excess water and a place to dump it. I used to work with a gentleman who put in a well water source heat pump and he dumped it into a creek about 100 yards in the back of his house. The county health dept said you couldn't dump it back in the well because it would contaminate the aquifer. The biggest problem he had with this system since he hooked it up to his drinking water well was the fact it brought the sulphur levels in the water way up over what a normal household pumping would have. I would take this into consideration along with the wear and tear on the well pump. Another thing you have to consider is upgrading the well pump to a commercial unit that was built to handle the load being put on it. I always wondered what the sulphur and the minerals in the water were doing to his heat exchanger in the heat pump.

"piping the discharge of the heat pump into the irrigation main" Not sure what you mean by this statement. Can you clarify. If you are just planning for the cooling load you may be able to just use a hydronic air handler directly and not use a water source heat pump. I have done just this with a radiant floor system. During the summer months I disconnect the heat source from the radiant panel and allow the irrigation system to flow thru. I get about a ton of cooling. I live in a dry part of the country and condensation has not been a problem.

You might want to to check with zoning authorities too. Technically, we were not allowed to discharge the water from the heat pump anywhere but back into the ground. We don't have a heat pump at our current house but I do miss it for the A/C it provided plus the water for the garden. We had a private contractor who put in a secondary piping that I could access at will. Luckily this was done after the initial inspection and no questions were asked.

It's been a while since there were any postings to your question about irrigation. What did you decide to do about this? I'm planning on installing a new geothermal system and had thought about using the discharge for irrigation as well. Would love to benefit from someone else's experience. (Just joined this site.)