I started a business and my net income was negative. Can I still make an IRA contribution (from my savings) and is it further deductible against my already negative NI?

You must have taxable compensation to contribute to an IRA.  Since you have a net loss from the self-employment you do not have taxable compensation and therefore cannot contribute to an IRA.

Thanks DoninGA! As a follow-up, I had taxable compensation from a prior job before starting this business (which exceeds the SE loss). Am I allowed to contribute to an IRA?

If you have taxable compensation on your 2017 return from other wages then you can contribute to an IRA.
The IRA contribution limit is $5,500 if under age 50 or the amount of your compensation if it less than the limit.