I take it from your question that you paid Class D PRSI ? On the other hand, if you paid at Class A you may be eligible for Illnesss Benefit or Invalidity Pension if you meet the relevant PRSI contribution criteria. Your local Social Welfare/Intreo office can advise.
2 As you retired on ill-health grounds I assume that you were granted notional added years for pension purposes. There are some differences between the various public service pension schemes but generally I am fairly sure that you will not be able to enter any pensionable employment in the public service and keep your current pension. Theoretically, if you are declaring yourself fit for employment (and this is subsequently verified) your ill-health retirement may be questioned.
I don't know what the situation might be as regards occasional temporary roles.