How do you pronounce your username ?

[Sirion]

Sirion was the main river in Beleriand during the First Age of Middle-earth. I think its name is sindarin. You just gotta check the phonology of sindarin and you'll get the answer! Easy enough!

Seriously though, I never really studied sindarin (or any other language created by Tolkien) but I always pronounced it "Seereeon" (which I believe is the actual spelling). But I've already heard people say "Saireeon" or "Seereeohn".

I hope I transcribed sounds well enough for you to understand. IF I had already learnt fonetics in college, I'd explain it better and with more details.