St Patrick's Cathedral

Ireland's most significant church
Noteworthy for: The cathedral is built next to the well where Saint Patrick baptised converts on his visit to Dublin.

Although built in honour of Ireland's patron saint in 1191, the country's national cathedral is today associated with a rather more contemporary countryman: Jonathan Swift, author of 'Gulliver's Travels', who was Dean here from 1713-1745.
Swift's is one of the many graves and memorial plaques you can see inside the building, which - vastness aside - isn't hugely architecturally distinguished. Nonetheless, if you like ancient churches, this should definitely be on your intinerary.

If you don't fancy paying the entrance fee, the gardens are a pleasant spot for a stroll.