Start with a use you can measure
Decide which containers or beds will use collected water. In an Irish garden, frequent rainfall does not guarantee a useful supply through every dry period. Measure your normal watering demand over several sessions and identify how far you would need to carry the water.
Estimate collection separately from storage
The rainwater calculator estimates potential collection from roof area, rainfall and an efficiency assumption. It does not choose the tank size for you. Compare a plausible dry spell with available storage and practical demand, rather than buying a barrel based only on annual rainfall.
Check the installation details
Use a suitable stable support and a secure lid. Plan access to the tap, cleaning and an overflow route that will not cause damp or inconvenience neighbours. Check the compatibility of the diverter with your downpipe and follow the installation instructions.
Treat it as garden water
Roof runoff should not be presented as drinking water. Keep the system separate from potable supplies and follow local requirements for connections. Reassess the barrel after heavy rain and a dry period: leakage, awkward access or unused water may tell you more than the capacity printed on its label.