Joey Maguire is one of the very briefly seen Maguire brothers, along with Donny (and possibly Fergal) that were featured early on in the show as guest characters, before the Maguire Family joined the primary cast, and became main characters. In which him and Donny were replaced with Maguire brothers that had more potential for personaity, character development and plot, such as Shane and Micky.
Joey's only credited appearance is in Episode 6 of Series 2 (even then he is never named on screen) as part of the Maguire family during their initiation of Ian into their clan after they think he has impregnated Mandy. Joey is generally portrayed as a menacing, tough, and violent character, but generally a mute who follows his father Paddy's orders, showing no voice, independent thought, or unique character traits whatsoever, fitting the archetype of a general thug/goon.
Another Maguire brother is seen with Donny, and another uncredited brother, in Series 1, when they attempted to attack Ian Gallagher 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. Although both this Maguire brother and Donny are only labelled Maguire 1 and Maguire 2 in the credits, it is assumed to be Joey (despite a different actor) due to how the same actor is retained for Donny a series later in series 2, when he is credited officially himself.
As well as being one of the Maguire boys first seen in the series, Joey may be identified as one of the 2 unnamed and uncredited Maguire brothers that accompany Donny and Paddy around in Series 3, as well as one of the extras that appear throughout Series 4 that are implied to be Maguire brothers, all being nothing more than followers without much (if any) dialogue. It's likely Joey can just be seen as a default Maguire brother used early on, for when Paddy or Mimi needed a goon. Fans generally tend to assume Joey is the third Maguire brother during scenes in Episode 8 of Series 4 involving Mimi, Shane and Micky praying before dinner, as well as the other brother at Jamie and Karen's wedding.
Usually Joey has been portrayed as the silent and obedient member of the family, even serving as the designated car driver, to escort his parents, in what little appearances he has been in. But after Series 4, none of the extra Maguire brothers appear (save for a brief 2 second clip in Episode 2 of Series 5 only showing 3 people kicking Inky Bob in the street, with a confirmation that one is Shane), and there's no mention as to Joey's wearabouts. In likely mistake from the writers, Mimi has forgotten to mention Joey when counting her children at certain points in the show.
The only actor credited as Joey, William Willoughby, has assisted Shameless in other ways apart from just his guest appearance as Maguire son. He worked as a stunt performer in Episode 13 of Series 7 and Episode 18 of Series 8. . http://www.imdb.com/name/nm0932598/.