The core requirements are that the home was built and occupied before 2021, you apply and get a grant offer from SEAI before any work starts, the work is done by an SEAI-registered installer, and a BER assessment is completed after the works. Once you have the offer, you have 8 months to finish the work. 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 the main exception: a home built or first occupied from 2021 on isn't eligible, so if you're building or have just built, you'll pay the full cost without the grant (up to €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.