I would go with the ¾ HP. It all depends on the depth to water level. If it gets anywhere close to 24’, the ½ HP would only produce about 1 GPM. The ¾ HP would still produce about 5 GPM. If the water level stays high, the ½ HP would work. But I would rather be safe than sorry.
Also I think “general garden and yard sprinklers” would be considered as irrigation.