Cillian is an Irish name, usually explained as "little church" from cill, church, with a diminutive ending, though some link it to ceallach, "strife"; St. Cillian was a 7th-century Irish missionary to Germany. The actor Cillian Murphy, who won the Oscar for Oppenheimer in 2024, taught Americans the silent C. Cillian had fewer than five US births in 2000, entered the top 1000 in 2020 and has climbed to number 507 in 2025, after a peak of number 463 in 2024.