For the SEAI grant you do need a BER, but the timing trips a lot of people up. Most grants only require a BER cert *after* the work is done, so you generally don't need to rush out and get one before you start — and you don't need a fresh BER just because an old one has expired, unless you're selling, renting, or applying for grants. The post-works BER reflects the improvement the solar (and any other upgrades) made to the home's rating.
If you're also chasing other measures — like a heat pump grant — the BER plays a bigger role up front, because the assessment can tell you what insulation or airtightness work you need to do first to qualify (for example bringing the home's heat loss below a threshold before a heat pump is grant-eligible). In that case the BER assessor's report becomes a roadmap: do the attic and wall insulation, improve airtightness, then the heat pump qualifies. For solar PV alone, it's less involved.
Practical advice: confirm with your installer whether the BER is included in the quote or a separate cost (it's sometimes excluded), and budget for it either way. For grant purposes the key BER is the one done after the works. If you're stacking solar with insulation or a heat pump, get the BER assessment early so it can guide the sequence of works and the grants you can claim.