You can usually add a battery later, but the decision you make now about the inverter is what determines how easy and cheap that will be. If you fit a hybrid (battery-ready) inverter at install time, adding storage down the line is a relatively straightforward job. If you go with a cheaper string inverter to save money up front, you'll likely need to replace the inverter when you add a battery, which eats into the saving.
There's also a VAT angle that catches people out. Batteries are zero-rated for VAT when bought as part of a solar PV installation, but if you add one later to an existing system it attracts the standard 13.5% rate. So if you're fairly sure you'll want storage, buying it with the panels can save you the VAT — and you don't need to qualify for any grant to get the 0% rate, you just need it to be part of a PV install.
A few Irish solar owners deliberately go battery-ready but skip the battery initially, watch their real generation and consumption for a season, then size the battery to what they actually see. That's a sensible, low-regret path — just budget for the hybrid inverter premium (often around €1k extra) so the later upgrade is painless.