Metered export is the electricity your smart meter actually measures flowing to the grid — you're paid on real readings. Deemed export is an *estimate* the system applies when you don't have a smart meter recording your export; it's calculated from your inverter size (a standard assumed amount) rather than from what you genuinely sent out. Both result in a payment, but one is based on reality and one on a formula.
Which is better for you depends on your export volume. If you generate a lot of surplus and export heavily, metered export typically pays more because you're credited for every actual unit. If you export only a little — say a small, high-self-consumption system — the deemed estimate can be more generous than your true output would earn, so a smart meter could actually reduce your export credit. This is exactly why some owners on older day/night meters hesitated to switch to a smart meter when ESB offered.
A wrinkle worth knowing: deemed export, once applied, generally isn't retrospectively adjusted — they don't go back and recalculate it later based on what your actual export turned out to be. The whole area has been in flux, with day/night-compatible smart meters and changing rules, so the deemed-vs-metered calculus can shift. If you're deciding whether to accept a smart meter, it's worth estimating your likely annual export first; high exporters generally gain from metering, low exporters sometimes don't.