These do the same end job — hot water — by very different means, and both are valid. Solar thermal panels heat water directly and are highly efficient at it, often around 80% efficient, compared to roughly 20% for a PV panel. The catch is that solar thermal is single-purpose: it only heats water, it can't power your house or charge a battery or an EV. It typically takes around four PV panels to heat as much water as a single solar thermal panel.
For most new installs, the better choice is PV plus a diverter rather than dedicated solar thermal, precisely because PV is multi-purpose. Every kWh of electricity is worth something — you either use it or sell it under CEG — whereas hot water just goes cold and is wasted if you don't use it. So putting most of your roof into PV (electricity) and heating water with surplus via a diverter usually gives more overall value than committing roof space to thermal.
That said, long-standing solar thermal systems — some 16+ years in — perform well: simple, reliable and hard to improve on. If you already have working solar thermal panels, the strong advice is to keep them and max out the rest of the roof with PV, rather than ripping out efficient thermal to replace it with PV. New from scratch, PV-plus-diverter is the more flexible default for Irish homes.