The real answer has not been given yet, it boils down to marketing. Offering flights for a penny sounds to good to be true for most people. It makes them curious and draws them to the Ryanair website. That is exactly what Ryanair wants, it is free publicity. Then for given flights there are a set of seats available for that price. It will be on the days that most people can't fly for a leasurly reason thus during weekdays or on flights that draw not that many customers.
But since people have been drawn to the website they stick around and try to find a cheap deal. Ryanair knows this and the whole website is set to encourage you to buy a flight, they state how many seats are available for the discounted price, encouraging you to keep searching for a cheap deal. In the end most people will settle for a reasonable cheap flight to a destination which was not there first choice but the flight was cheap so they bought it. ;)
As of now they state that they have 500.000 seats available for discounted price of 25% off, they have roughly 2000 departures a day, with 189 seats per plane this works out to 378.000 seats departing on a single day. the campaign runs for a month so that results in 11.3 million seats. In the end they have put 4.4% of there seats for that month on discount. Not that many, that is roughly 8 seats per aircraft or little over 1 row of seats. In the end it is just a very cheap marketing strategy!
Then the part of the question, How can they offer them so cheap? Willingness to pay is the key, last minute seats are expensive, since you obviously need to go somewhere in a hurry, this means that you are willing to spend more on a ticket. These customers pay for the cheaper tickets bought way in advance buy others. Routes with a high demand also allow the airline to ask more for a seat.