(2-part post)
The general security rules for hand luggage (which apply to all departures, with all airlines, from all EU airports) are here:
http://www.dft.gov.uk/transportforyou/airtrave l/airportsecurity/
Note, however, that Ryanair's maximum measurements for your single item of hand baggage are slightly less than the statutory ones. (Ojread2 has provided the relevant dimensions).
The list of prohibited items in cabin baggage (which applied even before the current security regulations came into effect) is here:
http://www.ryanair.com/site/EN/faqs.php?sect=b ag&quest=prohibiteditems
If you actually check in at the airport, the minimum check-in time is 40 minutes. There's a �2 fee, payable at the time of booking, for airport check-in.
However, there's no point in queuing at the airport check-in desks if you're only taking hand baggage. It's far easier to check in online. Online check-in is free and you also get priority boarding. You can check in online from 2 days before your flight, up until 4 hours before departure.
When you check in online, make sure that you've got your passport handy, as you'll have to enter the number.