The strongest, most repeated advice is to design the system around *your* needs first, then go looking for quotes — rather than letting a salesperson size it for their convenience. Homeowners are often steered towards panels on the wrong roof (sometimes the easier-to-install side rather than the better-producing one), so come in knowing roughly what orientation and size suits your house, and treat a quote that ignores that with caution.
Beyond the price and spec, weigh the things that don't appear on the quote: clear communication, realistic lead times, and — above all — after-sales support. Good post-installation support is consistently as important as the upfront figure, because solar systems occasionally need a setting tweaked, a fault chased, or a part swapped, and an installer who actually answers the phone afterwards is worth a small premium. Check reviews and, ideally, talk to local people who've used the company and ask specifically about how they handled problems and the SEAI grant process.
A practical filter: prefer installers who use their own crews over those who outsource to subcontractors, because subcontracted work has produced quality problems and finger-pointing when something goes wrong. Make sure the installer is SEAI-registered (needed for the grant), get three like-for-like quotes, and favour the ones with a track record of tidy, compliant work and no high-pressure sales tactics. Reputation and aftercare beat the lowest number.