1. I'd say that it is the 1st "bh" that is silent rather than the 2nd one: [ga-(ı)v]. As Jaime (almost) points out, the lenited consonants "bh", "mh", "gh" and "dh" are often vocalised when they occur between vowels or between a vowel and a consonant, which means that they are pronounced more like vowels than consonants, and they then merge and interact with the neighbouring real vowels, often lengthening them or creating diphthongs.
2. The literal meaning of "Gabhaibh mo leisgeul" is "Accept (or take) my excuse". "Gabhaibh" is the plural imperative form (for giving an order to more than one person) of the verb "gabh" which, amongst other things, means "take". "Mas e do thoil e", on the other hand, literally means "If it is your will".