I'm not entirely confident as to the slightly different shades meaning between "tá sé oscailte" (it is open) and tá sé ar oscailt" (it is open). But Foclóir.ie does give "oscailte" for "not shut" and both "oscailte" and "ar oscailt" for "available for use or access" (https://www.focloir.ie/en/dictionary/ei/open)
A verbal noun is "A noun that is derived from a verb and usually preserves the verb's syntactic features, such as transitivity or the capability of taking nominal or verbal complements." (The American Heritage® Dictionary of the English Language, 5th Edition. https://www.wordnik.com/words/verbal%20noun
In English, the gerund, for example
Noun: oscailt, f. (gs. <sub>e, pl. </sub>í) 2nd declension (f. and gs. ends in e or í)
Means "opening", is a verbal noun because it's derived from the verb "to open" OSCAIL.
Lovely usage eg of the noun from teanglann: Eye, mouth, door, a wound, a space between rocks, or the first strip ploughed.