"Irish Names for Children" Patrick Woulfe (1923)
Diarmaid/Dermot (Jeremiah, Jerome, Darby ..etc)
"generally angicised Jeremiah"
and in the 1920s, when this book was published, there was encouragement to christen children with more Gaelic names.
So some little Dermots were called after their grandads Jeremiah/Darby.
eadaoin