Question about EU citizen marrying an illegal?

[Duijin]

So I'm an EU citizen currently residing in my home country. My girlfriend is currently illegally residing in Spain. I've been trying to search for some information on what the rules and procedures are for EU citizens to marry illegal residents in Spain, but I've had some pretty conflicting information. What exactly are the rules? What documents do we need? Do I have to be working in Spain in order to be eligible? Also, is it easiet to marry in Spain or Portugal?

Would be grateful for answers