What to prepare for automatic irrigation

Please note, however, that you will then need different valves.

Or a power supply for the corresponding voltage.

Which valves would those be?

As Grantlhaua wrote, either use the same valves and power supply for the correct voltage and current or directly the valves that operate at the same voltage as the switching actuators (more expensive). You then have to assemble the control and visualization yourself in free@Home. There might also be ready-made modules. As I said, I only know a little about KNX.