Irish born, Siobhan O'Hara is not a subject of Her Majesty, the Queen, but rather one of those Rebels from the Irish Free State.  However, in the coming war beggars for men and machines will not have the opportunity to be picky when it comes to ablebodied souls anxious to fight.  Whether the War Office has figured that out yet, Colonel Nelson certainly has.  How he managed to get her into his unit, is something of a mystery.