Irish passport

I have applied for my irish passport parent both irish I was born in UK,

I have live in South Africa for over 25 years. I have been asked by the passport office for an umbridge marriage certificate, I sent my original cert that was sign and stamps by the magistrate count in Port Elizabeth. (My husband is South Arfican). Why do i need an umbridge cert when it start only original documents to be submitted.

How do I get an umbridge marriage cert from ireland.

Thanks Maria