If this arrangement will be your first time becoming a tenant then unfortunately you won't qualify for relief.
The credit is only available to people who had already been tenants prior to December 2010.
If you were a tenant who would have been entitled to the credit at that date, then your entitlement moves with you to subsequent tenancies.
To answer your other questions, you can be renting from the existing tenant or the landlord, that makes no difference, and if the monthly rent is €300 then you would qualify for the maximum credit, which is €320 (this is the amount of tax credit i.e. the amount of tax you save).