The only Harriet Campbell born Co.Tyrone with father named George that I can find is the Harriot born 1866 which would be 10 years off from 1901 census age. However, I'm inclined to think this is her.
Where did you get the information that George Campbell was from Ballyveeny? Mary Ann's marriage certificate? if so. remember that the residence listed is for the bride (or groom) at the time of their marriage and may not be the place they were born or where their family was living.