If you had any income or capital gains for the income tax year ended 5 April 2016, then you should have told the Revenue by 5 October 2016 so they get an opportunity to send you a tax return. This tax return, if it is on paper, would need to be filed by 31 October 2016. If you miss these deadlines then in theory you can be fined. In practice you should just go and see accountants such as David Porthouse and Co and not worry. If we are given warning, with any time in November being more than ample, then we can get your tax return filed by the electronic deadline of 31 January 2017.
You could in principle do your own electronic filing, in which case your electronic filing deadline would also be 31 January 2017. However, if you have left things this long then that is a good hint that it is time to engage some accountants.