No relation but... if Mrs. Frances Kelly was Joseph's sister, would she not have been Mrs. something else (unless she married a Kelly)? Or perhaps Frances was his sister-in-law, and not sister?
Was Frances born in Portland too?
Do you know when she came to Ireland, by any chance? Was she Mrs. Frances Kelly then? What was her approx age? Can you find her marriage cert? Who was her spouse?
The only records from the 1930's available online that I know of are the Dublin Electoral Rolls http://dublinheritage.ie/electoral/search.php or perhaps births on familysearch.org for Kelly, but parents names would be needed, of course.
Good luck.