The show, created by Jimmy McGovern, will return with four more episodes next year.
The first instalment stars former Shameless actress Anne-Marie Duff and Peep Show's Olivia Colman as mothers determined to do the right thing in a fight against crime.
The sons will be played by Waterloo Road's Oliver Lee and Thomas Brodie-Sangster, who has appeared in movies such as Love Actually and Nanny McPhee.
The episode will also star former Skins actor Joe Dempsie as a fearsome gang leader.
Meanwhile, Sheehan will appear in the second episode as a teenager concerned about the palliative care given to his mother by her nurse, played by Smith.
Sheehan's character Stephen becomes increasingly worried about his brother Dom (Josh Bolt) and begins to argue with his father, played by comedian John Bishop.
Filming on the first two episodes is due to be completed before Christmas, while the second pair will be finished by February.
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