However, here are some possible beginnings depending on the intended meaning:
* Is mise an rud a... (Is mise an rud a bhíonn... for habitual actions) This translates to "I am the thing that..." or "I am that which..." This works if "what" refers to a noun or a thing.
* Is mise an té a... This translates to "I am the one who..." This is suitable if "what" refers to a person or a role.
* 'S mise é/í a... (Depending on gender) This is a more colloquial way of saying "It's me that..." or "I'm the one who..."
To get a more accurate translation, please provide the complete sentence.