mh is pronounced v, w or silent.
"A symbol called a sí buailte (she boo-ilta) was once used to change the sound of some consonants in written Irish. With the advent of the typewriter, the letter h was substituted for this purpose, but often retains the Irish name."
See the bottom of the following page:
http://www.mustrad.org.uk/reviews/irish.htm
Here you can hear two versions:
http://www.babynamesofireland.com/pages/girl-names-n-z.html