From the information given, you will have difficulty funding an IRA from rental income alone. As mentioned, contributing to IRAs/401Ks/SEPs means you have to have earned income (compensation) during the year. Rental income alone is not considered earned income.
Here is a definition of earned income (compensation):
Compensation does not include earnings and profits from property, such as rental income, interest and dividend income, or any amount received as pension or annuity income, or as deferred compensation.
http://www.irs.gov/taxtopics/tc451.htmlNow, where you can get creative depends upon how you setup your business. If you have a 5-plex and you plan on adding more properties, you could consider creating a property management company to cater to your property (collect rents, maintenance, ect). This could be setup as a separate entity that charges market rates. From this income, you could fund retirement funds as your provide a service; however, this is a new business separate from your rental property's LLC.
Rental property income is difficult to shelter into tax-deferred accounts, unless you purchase the property through one of these accounts. There are self-directed IRAs used to purchase real estate, but you already own the properties and you seem to be more interested in translating rental income to earned income to save on taxes.
As with the overall advice, I would get a CPA. Paying for an hourly consultation most likely will result in generalist advice. A good CPA should be able to look at your financial life and make an honest assessment of adjustments you could make. It pays long-term and is a smart expense to take on.