"What is going on?"
Translation:Cad atá ar siúl?
Please correct me if I'm wrong here.
From my understanding, "Cad é" is when you are asking questions using the copula built into "cad", e.g. "what is it?". "Cad atá..." is for situations where you are not using the copula for your question and the copula is implied, e.g. "Cad atá ar siúl", which can be thought of as implying the "é", i.e. "Cad [é] atá ar siúl" meaning "What [is it] is going on", or more naturally "what [is it] that is going on".
There are two different type of questions in English and Irish. Questions about actions involve a verb - "do you eat cheese?" - an itheann tú cáis?, "Are you ready?" - An bhfuil tú réidh?, "Does this bus go to Cork?" - An dtéann an bus seo go Corcaigh?.
In English, these question either use "do" or they reverse the subject and verb ("I am" -> "Am I?"). In Irish they involve the interrogative particle an, or the interrogative form of the copula (which is also an).
Then there are "W" question - "who", "what", "why", "when", "where" and "how". In Irish, these are C questions - cé, cad, cén fáth, cathain, cá and conas. There isn't always a one to one correspondence, and different forms of questions may be preferred in different dialects, but, to answer your question, no, most questions do not involve bhfuil.
The interrogatives cé = "who", cad = "what", céard = "what", conas = "how" require a direct relative clause (atá).
The interrogatives cá = "where", cén fath = "why", cén chaoi = "how" , as well as the combinations with prepositional pronouns cé/cad leis = "with what", cé/cad air = "on what", etc. require an indirect relative clause (go/a bhfuil).
Cad atá ag tarlú could also be used here, an interjection to ask what's happening, what's up, what's going on, how someone is. Tarlaigh as a verb means to happen, to ensue, to arise, to occur. So, any Gavin & Stacey fans, or indeed any Welsh people would recognise "what's occurring" as meaning "what's going on" or "what's up?"
Apart from the issue of whether or not céard has been added as an additional answer for this exercise, note that ceard without a fada means an artisan or skilled tradesman, and is used in compound words such as ceardchumann - "trade union" and ceardlann - "workshop".