The answer to this depends upon whose journey you are focusing.
In relation to Amir's journey to find Sorhab, it is situationally ironic that it is Assef keeping Sorhab "prisoner". The same man who raped Hassan is now raping his son--Assef sought Sorhab out on purpose in order to exact his revenge from their childhood.
It is also situationally ironic that Sorhab could not be adopted legally from Afghanistan, but if Amir could just get him to the country (illegally!), the adoption could take place on US soil.
It is also ironic that it is Sorhab and the slingshot that save Amir from Assef's brutality just as Hassan and his slingshot had saved him in the past.
As for dramatic irony, if you are discussing Amir's and Baba's journey as father and son, you could talk about how Baba thinks that Assef is fantastic, when in reality he's a sociopath.