We may be back to the problem with the I.G.I. that many illegitimate children are indexed under the mother's surname, but may have been known by the father's.
The answer is to look at the microfilm of the register transcript + see whether there is a baptism of a suitable Margaret.
Alternatively, her family may have moved to Ballaugh when she was very young + she thought that she was born there. Therefore look for a family having Mylchreest children there a few years later.