Hi there and welcome to the forum,
I taught English in Spain from Sept 2012 to Dec 2017, so should be able to answer any questions you have. :)
The first thing to know is that jobs in Spain are not typically found from abroad. There are so many teachers in Spain already that employers have no need to take a chance on somebody who's not already in the country. You really need to be on the ground at the right time handing out CVs. The peak hiring time is September, and there's a smaller hiring peak in very early January to replace teachers who jumped ship at Xmas. While it is possible to find work outside of these times, you are likely to have to piece together a timetable by working at two or more schools, or by taking on private classes.
Note that not having the right documentation already in place (NIE, social security number, and Certificado de Delitos de Naturaleza Sexual) will put employers off. I don't know where you're from, but if you're British/Irish, I'd strongly recommend getting the NIE while still in your home country. In the UK, this can be done in London or Edinburgh (https://www.citizensadvice.org.es/faq/n ... edinburgh/
). In Ireland, I'm going to guess that it's Dublin.
If you're a non-EU citizen with no right to a passport from an EU member state, either through birth or ancestry, I'm afraid your chances of finding legal work in Spain are slim to none. This is because EU hiring law dictates that in order to hire a non-EU citizen, employers first have to prove that there were no suitably-qualified EU citizens who could do the job. When it comes to teaching English, this is not a very likely proposition.
Hope that helps, and if you have any other questions, please don't hesitate to ask.