Simply sign up to the Ryanair subscribers' list and you'll find out when all their offers are taking place:
http://www.ryanair.com/site/EN/about.php?page= About?page=About&sec=register
The best offers which Ryanair currently have are part of their Autumn sale, which ends at midnight tonight. Many flights, booked by that time, (to a wide variety of destinations, including Dublin) are available at �10 each way, including taxes and charges. The flights have to be taken between 05/11/07 and 07/02/08 (Monday to Thursday only). The period 18/12/07 to 08/01/08 is excluded.
Chris
PS: The only true tax, on UK routes to Dublin, is the �10 tax which the British government charges for flights leaving the UK. Much of the rest of Ryanair's 'taxes & charges' is made up of their 'passenger service charge':
http://www.independent.ie/national-news/passen gers-paying-17m-in-ryanair-phantom-tax-87384.h tml