If you had any income taxed under PAYE, tick the box. It will restrict the relief automatically if the amount is less than sufficient to use the full credit.
Re question 1 What part of the form are you looking at ?
Are you doing the return for 2016. It is likely that details under earned income from your p60 have not been prepopulated yet. Because PRSI is not strictly speaking tax, it is not possible to include details without the p60, which shows how many weeks at which rate.
I cannot see how you can manually enter details of PRSI from the p60 yourself. Maybe some other poster can comment.
Re question 3. Yes I think that is the usual credit, it is a credit not an allowance, if you are going to do the returns yourself you should read up on the difference.