With the risk of offending some people here, the following has been my experience, which has cost me money. I am not a technical guy qualified in this area, only what experience has taught me.
In my experience an SEAI approved installer or supplier for that matter doesn't matter a hill of beans. Once someone goes off and does a day's course they are approved, SEAI says all is ok. When it comes to solar heating, you get what you pay for. Compare the output (all systems are measured against a standard) between the various suppliers and it is pretty eye opening, the variations are massive. In my experience there are only a couple of systems with high output and they are the more expensive options. I now have tubes and while very happy with them and the performance, I do query the payback. Obviously hitting high temps these days. Did you not consider looking at pv electric panels and connect into your DHW cylinder? Just a thought, but I don't know much about pv systems. all the best.