If you want the Revenue Commissioners to calculate the tax for you, you need to submit your tax return by 31 August 2015.
Revenue e-brief No. 74/15 provides"If a customer files their Form 11 after 31 August 2015 they must make their own self-assessment and calculate the tax, PRSI and USC due by 31 October 2015."
The final deadline to pay and file is 31 October 2015. However if you both pay and file online using ROS the deadline is 12 November 2015.
For further information see: Revenue e-Brief 74/15