Gale force winds and torrential rain have begun striking both countries - but should skirt around the royal wedding in Windsor.
The Met Office has warned heavy rainfall could present a risk to life.
Around 950 homes in south West England and nearly 100 in south Wales have also lost electricity.
Winds of 76mph on the Isles of Scilly were the highest recorded, while parts of Cornwall and Pembrokshire experienced 60mph gales.
Cardiff and Exeter airports also announced some flights had been cancelled because of the weather.
The deluge poses a risk of communities being cut off by flooding.
Mudslides were reported in Cork and water "overtopping" at docks in Galway.
A series of weather warnings have been issued by the Met Office, stretching from north west coast of Scotland to Land's End.
Wales has been put on yellow and amber warnings, braced for between 5cm and 15cm of rain.
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Guests arriving at the royal wedding in a windy Windsor had to hold on to their hats, with gusts of up to 32 mph.
But it is set to escape the worst of Storm Callum, remaining "quite dry" and with temperatures up to 24C (75F).