Hi @micahberghan,
Welcome to the Bunnings Workshop community, it is fantastic to have you with us.
When building fences, you should always concrete the posts directly into the ground. If you use H4-treated posts, which are treated against boring insects such as termites, and coat the underground portion of them with bitumen membrane, they will be fine. For a 1200mm high fence, you should use 1800mm posts so that 600mm is buried underground.
The reason I would not recommend stirrups is that they are designed for vertical support, not lateral support. When you add palings to a fence, you create what is essentially a wind sail, so when the wind blows, a surprising amount of lateral force is applied to a fence.
If you were to use stirrups, this lateral load will be transferred down the posts and be focused on the spot where the post is bolted to the stirrup. This force, being applied to a very localised area, over an extended period of time, will eventually cause the timber or bolts to snap.
When the post is concreted into the ground, the force is spread out over a much wider area, meaning there is much more surface area for that force to dissipate into the concrete. Long term, it is a much better solution than using stirrups.
Let me know what you think and if you have further questions, please don't hesitate to ask.
Jacob
