I'm not sure if you are asking for pronunciation or usage, so I will answer both.
In AE at least, godless sounds like "God-liss", or "God-less".
I do not hear it very often in conversations, but I have seen it in books from time to time. People know what it means, I suppose the opportunity to say it doesn't arise very often. Religion is a very touchy subject for discussion.
That being said, I hear "irreverant" much more often than godless.
Hello, Stele. As a person with no religious beliefs, I am godless and wouldn't mind if somebody were to describe me that way. As Var18 mentioned, religion can be a difficult subject for some here in the US. Many people profess many different beliefs over here, and we natives often avoid adjectives that others may find insulting. I would guess that "godless" would be used more often among the faithful to describe those who don't share their beliefs.
You have received two good answers, or as good as they can be without any context.
You should always give us context. In this case you could give us a sentence where you may way want to use the word, or the sentence where you have seen it. In that way, people can tell you whether it fits, and whether it says what you have in mind.
The context in which I have most often heard it in my lifetime is in the phrase "godless communism", as most countries that were officially Communist were also officially both atheistic, and actively hostile to religious belief (e.g., Russia under Stalin.)