그럴 리가 없다 - There is no way that it will happen. 그럴 리가 없대 - They say that there is no way that it will happen. The 대 ending means that the speaker heard someone else say it, or read it somewhere (i.e. not the speaker's own words)

그럴 리가 없대 is the shortened form of 그럴 리가 없다고 해. 그럴 리가 없다(it is not likely~) +(라)고 해(someone told you some information of the contradiction ) it is that you express 'it's not likely~/ it's not likely to happen' because you knew about the information by talking of other people and you indirectly reaveal such the situation by '~(라)고 해'

그럴 리가 없다. 리(理), a reason or cause There is no reason ....