Simply.. what is the difference between these two sentences:

Tá an seomra seo níos dorcha.

Is dorcha an seomra seo.

When can I use which of them?

Tá an seomra seo níos dorcha.

= This room is darker.

Is dorcha an seomra (é) seo.

= This is a dark room. (emphasis on "dark")

Unfortunately dorcha has no marked comparative.
So "Is dorcha an seomra seo" is clearly "This is a dark room".
Let's use another adjective, e.g. maith
But "Is fearr an seomra seo" is not correct because "ná" is missing:
"Is fearr an seomra seo ná an seomra sin = This room is better than that room"
"Is dorcha an seomra seo ná an seomra sin = This room is darker than that room"
And that's the answer to your second question: You can use the first without ná but you can't use the second without ná.

Lars

Thanks both of you!

Of course you can use the second one without 'ná'. When you want to give a comparasion without having to say 'ná an seomra sin' the whole time, you can just say 'is fearr an seomra seo'.

I think what norwegiandame was getting at was the comparative steps don't always have the 'níos' and 'is' present, but this is only when it's obvious that the adjective used is the comparative step. E.g. Is fearr é an seomra seo, Is deise é an seomra seo, etc.