Cillian probably from Gaelic ceall "church" combined with a diminutive suffix. This was the name of a 7th-century Irish saint who evangelized in Franconia. He was martyred in Würzburg.
Saintly namesake: Saint Kilian was a 7th-century Irish missionary bishop and martyr, venerated as the apostle of Franconia. He is one of several saints named Cillian or Kilian. (July 8 · 7th century)
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