The core requirements are that the property is an eligible home, the work is done by an SEAI-registered installer, and a BER assessment is completed as part of the process. The registered installer is key — they handle the bulk of the grant application and ensure the install meets SEAI's standards, which is one reason established, registered companies are the safer bet over the cheapest no-name option.
New builds are a notable exception: very new homes are generally not eligible for the solar grant, so if you're building or have just built, factor in that you'll likely be paying the full cost without the €1,800 off. Existing homes that meet SEAI's eligibility criteria are the typical grant recipients. The BER requirement means you'll need an assessment (usually after the works), and you should keep that cost in mind whether or not it's bundled into your installer's quote.
A practical tip: make sure your installer issues the NC6 microgenerator form to ESB Networks — ideally well ahead of the install date. That's part of getting you registered as a microgenerator (which matters for export payments) and is separate from, but runs alongside, the grant process. When comparing quotes, confirm the installer is SEAI-registered, that grant paperwork is included, and whether the BER is in the price or extra.