"Is it open?"
Translation:An bhfuil sé oscailte?
"Is it open?" is "An bhfuil sé ar oscailt".
An bhfuil sé oscailte? = Is it opened? (Has it been opened?, roughly)
"An bhfuil sé ar oscailt"
I have just tried "ar oscailt" to see if it was accepted: it was. (2019/08/10)
Hardly surprising - it's about 4 and a half years since those comments were posted!
Questions concerning predicative adjectives will almost always use a form of bí (e.g. an bhfuil ) rather than a form of is (e.g. an ).
oscailte é is nonsense. oscailte is an adjective, so it goes like any other adjectives An bhfuil sé oscailte? (which actually means "is it opened" not open, as a store would be.