You should get a Social Welfare medical cert from your GP (the first cert includes the application form) and send it to SW. You should get a SW medical cert for each week you're out sick, and then a final cert when you're ready to return to work.
If you have enough PRSI contributions, and as you say, you earn over €300 pw, your personal rate of Illness Benefit is €196. There are increases for qualified adults and qualified children.
Whether your employer pays you will depend on the terms and conditions of your employment - some pay, some don't.