I am also descended of the Leonards of County Dublin, Ireland. My 2 great grandmother Ellen Leonard was born in Rush (daughter of Nicholas Leonard and Bridget Flynn). Ellen married John Fulham of Portrane. They emigrated to Boston in 1864 where Ellen's Leonard uncles Christopher, Richard and eventually her parents settled. Most of our Leonards of Boston were seafarers. I imagine if you can pull your family back far enough, our ancestors cross paths. I know many of my ancestors traveled back to Ireland to visit with family (though none in my generation) The last time was during the 1960s. I believe that the Leonards of Portrane were neighbors of my Fulham ancestors.
Have you reviewed church records for Donabate parish? How far back can you take your line?