ruh mah agad
* ruh: The 'r' is a guttural sound, similar to the 'r' in a Scottish or French accent. The 'u' is like the 'u' in "put."
* mah: The 'a' is like the 'a' in "father."
* agad: The 'a' is again like the 'a' in "father," the 'g' is a soft 'g' as in "gentle," and the 'd' is a soft 'd'.
It's difficult to perfectly convey pronunciation through text, as Gaelic sounds have nuances not easily captured. Listening to a native speaker is the best way to learn the correct pronunciation. You can search online for audio examples of the phrase.