His first appearance in Series 1 (only credited as Maguire 1) involved him and two other Maguire brothers, forming a lynch mob after Ian, in revenge for trying to force himself on Mandy. However Mandy had lied to them, Ian had offended her by doing the opposite, and been unable to have sex with her due to being secretly gay. Donny later appeared again in Series 2 (now credited as Donny, as well as named on screen by his father) as part of the Maguire family, during their initiation of Ian into their clan, who they believed to have inpregnated Mandy. Donny later appears in Series 3, this time twice. In one episode, he joins his father and his two brothers in preventing Lip from seeing Katie Maguire, as well as capturing and torturing a thief who stole something of Mimi's. In another episode, he and Shane are ordered by Mimi to hunt down Carl after presuming he informed on them to the police.
Donny was generally confident, bold, somewhat sadistic and sociopathic (much like his brother Jamie in many regards), and extremely loyal to his family (given how protective he was over Mandy, and how he followed his parent's orders such as the way him and Paddy interact with each other in Series 2, and both parents as part of Series 3). Because he was portrayed as Paddy's reliable 'go-to-guy' and second in command, he seemed to be the highest ranking Maguire child in their family business at the time, especially the way the other Maguire brothers followed Donny around in Series 1, and how Donny would remain the most vocal out of the others. He also seemed to be the only Maguire brother, during Series 2 -3, that Paddy actually named and referenced directly.
In Episode 5 of Series 3 (the first time we see Shane Maguire), it became unclear from that point who between Donny and Shane was in charge. But as Mimi was mostly talking to Shane (during their scenes together) about the family business, with Donny just hanging back, and another scene where Shane was the one to threaten an under-aged drug-dealer (at knife point), Shane seemed to be higher in command, and the other brothers were absent so Donny served the role of the follower in contrast. Due to this, its possible that with Shane as the second oldest (after Jamie), Donny may be the third oldest Maguire son, who looks up to Shane, and upon Shane's entry he seemed to be phased out as a character afterward.
A couple of extras accomply Paddy, Mimi, Shane and Micky in Series 4 in various situations, such as meeting Lip and Mandy at The Jockey, accomplying the family to Jamie's prison release/welcome back party, Fergal's funeral, and Karen's introduction to the Maguire parents. They are generally assumed to be Donny and Joey, although it is never referenced or credited. Donny disappears soon after, with his current whereabouts unknown and just like Joey, Mimi has even failed to count him when talking about how many children she has (though this is likely due to a writer's error). However, unlike Joey, Donny has remained credited (mostly directly) in every pre-Series 4 appearance he has appeared in, and has appeared more often. Donny can also be identified in his appearances, despite various different actors, by what appears to be his trademark gold necklace, which all portrayals of him seem to share.