I'm planning a company charity ride to Dublin and was wondering how hard the route was and how many days to expect to take?
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It seems likely, since that is exactly NCN 4 - however, I haven't found any references to people riding the length of route 4, and the CTC route database isn't giving me anything either.
For reference, I plugged Charing Cross to Fishguard into cyclestreets and it reckons a shade under 300 miles (for the balanced route). Whether that means three consecutive 100-mile days, or five 60-mile days will depend on you.
I'd take the cyclestreets routes with a grain of salt, or at least study them carefully (and compare with the sustrans route description and mapping linked above).
Oh, and I see about 2km of total ascent on that route, with what look like some steepish sections once you're into Wales.