Occasionally in the early series (Series 1 to Series 4), before the Maguire Family were established into the main cast as a status quo, away from their antagonist roots, and often before Jamie, Shane and Micky even debuted, the family would usually consist of Paddy, Mimi, Mandy, Donny and Joey. But they would usually be shown with three goons considered sons, despite only two (Donny, Joey) being credited/referred to.
While these could be assumed to just be early incarnations of Shane and Micky (or possibly even Fergal), that remained unnamed for seemly spoiler reasons (or just the writer's not giving it much heed), Series 4 is notorious for including actors (or extras) for all six Maguire sons in some format, despite one additional goon that, if logic dictates, should make up a seventh son. He is likely just a close friend of the Maguire brothers, as he seems to be seen commonly alongside the extra assumed to be Donny, as well as following Shane around, as he is seen accomplying the entire family to Jamie's release from prison, plus he is seen distressing over the news of Fergal's death. However, the show divulges absolutely no information about him directly, as well as it being unknown why the show felt the need to have 3 extras to represent only 2 sons, and the person playing him seems to disappear after Episode 6 of Series 4.
It is possible though, that he is just a goon or hired muscle, recruited by Paddy or one of his sons, as repeated in later episodes like Episode 10 of Series 7 and Episode 9 of Series 10. However, due to his recurring Series 4 appearances in more than just one episode, as well as his presence in important family events, it's likely that he is part of the family, just not a brother/son.