This is a delicate and much mis understood subject.
In the UK a grant of a coat of arms is made by the College of Heralds (separate colleges for England, Scotland, Ireland) to an INDIVIDUAL, not to a family. The arms are normally inherited by the eldest son (there are variations depending on the specific grounds of the original grant).

There never was a College of Arms for the Isle of Man, and hence no specifically Manx arms were ever granted.

Some English persons with Manx surnames and some Manx persons have been granted arms by one of the Colleges.

Even though one may trace a direct descent from a person who has been granted Arms does not necessarily entitle you to display or use the Arms.