I used to be lukewarm about this name; however, the actor
Cillian Murphy has won me over to the I-like-and-appreciate-it side. Sometimes, all it takes is a positive association!
I will admit that, in English,
Cillian isn't the most intuitive spelling, but, with that said, I dislike
Killian because of the "kill-" part of the name. To me, it looks like "kill
Ian". Thus,
Cillian is the spelling I would choose.