The security rules relating to cabin baggage can be found here:
http://www.dft.gov.uk/transportforyou/airtrave l/airportsecurity/
However, you should note that Ryanair's size limits are slightly smaller than the standard ones:
http://www.ryanair.com/site/EN/faqs.php?sect=b ag&quest=cabinbaggageallowance
The 10Kg limit on cabin baggage is in addition to the allowance for hold baggage but, given the size limitations, you'd probably find it hard to get anywhere near to that weight in your flight bag. (In practice, while the dimensions of flight bags might be checked, they're hardly ever weighed).
The general list of items which are prohibited from the cabin can be viewed here:
http://www.ryanair.com/site/EN/faqs.php?sect=b ag&quest=prohibiteditems
For completeness, here's a list of the items you can't place in the hold:
http://www.ryanair.com/site/EN/faqs.php?sect=b ag&quest=prohibitedchecked
Chris