I like it. It's just a variant of Cillian, popular in France. Lots of popular British names are actually variants of names that originated in other countries. Plus Killian is a popular and very accepted version used in Ireland, and you avoid the 'kill' at the start of the name.
I know a young Cillian, and I live in England. His elder brothers friends think he has the coolest name ever (presumably due to the footballer) so you'd be choosing a name looked down on by your average middle class mum, but probably pretty cool amongst his peers. Who do you think he will care most about 😂