Question: A man divorced his wife one irrevocable Talaaq. Can he reunite with her while she is in her Iddah(waiting period)?

Answer: Yes, he can reunite with through a new Nikaah. She doesn’t have to wait for the Iddah period to elapse.
وإذا كان الطلاق بأنناً دون الثلاث فله ان يتزوجها في عدتها وبعد انقضاء عدتها ( اللباب في شرح الكتاب ج٢ ص ١٨٣)

And Allah Ta’ala Knows Best 

Yusuf Moosagie