Cillian \ci(l)-lian\ as a boy's name is of Irish and Gaelic origin, and the meaning of Cillian is "bright-headed". Also possibly "strife" or "monastery, church". See Kelly. Name of various early Irish saints. Killian is the anglicized form.
Baby names that sound like Cillian are Selwinn, Selwin and Sloan. Other similar baby names are Gillian, Callan, Collin, Cullin, Cullan, Dillan, Gillean, Gillion, Gillan, Gilian, Killean and Kilian.