MacCarthy was born in Southwark, London to Catholic Irish parents who had emigrated from Ballygarvan, County Cork.
In conjunction with two of his sons, Liam commissioned the manufacture of a trophy in the form of a mether, an ancient Irish drinking cup.
This cup was offered to the GAA central council in Croke Park and was gratefully accepted.
Liam became a leader in London's Irish community and was elected to the council of the Metropolitan Borough of Camberwell in the North Pechkam ward.
In 1915, Britain introduced conscription; a crisis meeting of the London branch of the Irish volunteers was held at his home in east Dulwich to discuss the issue.