The swedish _name_ Sirén cannot be translated at all, since it has no meaning. There are many swedish names of thís kind, bi-syllabic sounds without denotation. The swedish _word_ "siren", on the other hand, is a greek loan-word, denoting 1. a mythological figure. 2. An alarm horn. English language has this loan-word aswell, "siren".
The swedish word "syren" does mean "lilac" in English. But "syren" and "siren" are two different words, just as "Syrén" and "Sirén" are two different names.