According to this argument, Donnchadh's son and intended heir was Cailean (alias Nicholaus of Carrick), who as his son and heir, issued a charter in Donnchadh's lifetime, but seemingly predeceased him.
This argument suggests that Donnchadh's son and designated heir was Cailean (alias Nicholaus of Carrick). As the intended successor, Cailean issued a charter while his father was still alive but seems to have predeceased him.