Section 50 properties are Student Accommodation properties which also are allowed to be let out during the summer months as holiday homes. The relief you get is a write off of qualifying expenditure generally between 80 to 99% of the purchase price. Similar condition to what you may know as Section 23, i.e. must retain for 10 years, etc.
What they are saying is that you have qualifying expenditure of €280,000 which you can write off against Case V (Rental Income). This may just be the qualifying expenditure on the property, you may have additional captial allowance for Fixtures and Fittings. The €11,000 per annum guaranteed rent probably is gross rent - if this was net rent, then you would only use up €110,000 in 10 years so you probably would need to retain the property for a longer period or have other rental income to offset against. Check also is the €11k just the rental income over the 39 weeks (academic year).
Bit of advice on these types, check out the manageent agreement particularly for details of student deposits, damages,etc. A few clients have rued the day they bought one of these, the clients who do not have too many problems with them are the one where the management agreements covered everything and was managed actively.
Good luck.