From Irish Pedigrees; or the Origin and Stem of the Irish Nation by John O'Hart
OWEN (Eoghan), brother of John who is No. 118 on the "Hickey" pedigree, was the ancestor of O'Slatiairaidh; anglicised Slattery.
118. Owen O'Hickey: son of John; was called An-Slat-Iairaidh ("Slat": Irish, a rod, and "iair," to ask), as if he deserved the birch; a quo O'Slatiairaidh.
119. William O'Slattery: his son; was the first that assumed this sirname.
120. John O'Slattery: his son.