My birth certificate bears the name Brendain O'Glacain with a fatha over the "a" in Brendan and the second "a" in Glacain.
This indicates that the letter under it is long (drawn out) when spoken.Just like the a in drawn.
When Irish started to be written with english letters the fatha went as did the buncha( a dot over a letter) which indicated that the letter under it should be silent. This was replaced by the letter "h". Thus Gallagher is spoken with the second "g" silent. We therefore have "her" rather than "ger".