31 October 2018 is the deadline to file paper tax returns for the year ended 5 April 2018. If you know that you need to file a tax return, and you have done nothing yet, then it would be best to see an accountant at once if you have left it this long. The accountant can file an electronic tax return by the extended deadline of 31 January 2019.
You need to file a tax return if you had income for the year ended 5 April 2018 that the Revenue don’t know about, or if you had any capital gains for the same year. Income usually comes from starting self-employment, but there are other possibilities.