Sé is not a diminutive of Séaghdha, it is instead a variant from after the spelling reform movement: http://en.wikipedia.org/wiki/Irish_orthography#Spelling_reformSé & Séaghdha are pronounced identically, so even possible use as a diminutive is unlikely.MacLysaght includes it in an appendix of “Surnames of Ireland, ” one in a list of names stripped of their definition-origins by the reforms (that is, Séaghdha means hawk, Sé means nothing). [noted -ed]