In some ways this is more of a conlang question, but what RonJohn says in his comment is essentially correct; the people around Sirius won't call their sun Sirius, they'll call it 'the sun'. English at least tend towards reductionism and contract commonly used words so that they're easier to say faster so you can get your message across quickly.
Take for instance CAR. Horseless Carriage -> Carriage -> Car.
(also Chariot -> Car, and Train Carriage -> Carriage -> Car for less modern utilisations of this contraction; thanks James)
Or if you prefer, Facsimile Transmission -> Facsimile -> Fax. Words that we use more often will eventually have the shortest form possible. Hot. Cold. Hard. Soft. Tall. Short. Dog. Cat. Moon. Sun. The list goes on.
Not all languages seem to follow this model. Spanish doesn't seem to follow this trend for some reason. With words like Caliente, Adelante, etc. this trend of contraction doesn't seem to follow. Greek also seems to have longer words in common usage. Chinese (from the little I know of that language) seems to have a different approach completely; keep ALL words short, and use vocal inflection to differentiate between words rather than infer punctuation. Words like Wong. and Wong? in Chinese are actually different words, rather than the same word with the change in pitch reflective of it being a question or not.
So; the Spanish for Sunlight is Luz del Sol, which in its own way is every bit as clunky as Siriuslight, but then the Greek version looks to be even more complicated (according to Google; I'm not going to try to pronounce it) and ultimately, we have to remember that unless it's a human colony, there's nothing to say that the pronunciation of 'Sunlight' doesn't take place with a pheromone sequence or similar instead of the generation of sound waves.
As an aside, here in Australia we have a ex-mining town in Victoria called Bendigo. When it was first settled during Australia's Gold Rush, the town was supposed to be named after one of the 'Three Faithful Servants' of a story out of the Old Testament (Book of Daniel or Ezekiel if I remember correctly); Shadrach, Meshach, and Abednego. No-one could pronounce Abednego correctly, so it fell into 'Bendigo' in general parlance and that was how the name finally came to be known.
Know anyone named Felicity? What you might not know is that Felicity is a legitimate (yet now uncommon) word for great joy or happiness. But it's 4 syllables so most people just stopped using it in favour of Joy. Or happiness, which although it's 3 syllables so doesn't represent that much of an improvement, was based on a common word so people understood it.
The key point here is that words that are cumbersome to use and yet have common application tend to resolve themselves in time if it's a priority for the speakers.
So; your 'Siriuns' will find their own word that describes what they see in a manner that they're comfortable with. That said, 'Sirlight' doesn't sound too bad as an option.