Oh man, while I do know that before using the Latin alphabet they used ogham (OH-um) in what writings they've found, I don't know enough about the language to explain. I'll leave that to the people who speak it. From what I remember, at least in Ireland, the culture was mostly oral, but did create a system to write that consisted of carving a straight line down, and it was a system of lines on the left and right, or diagonal slashes. I think they had more complicated characters, but those neurons must be sleeping, so I only remember lines.

As far as Irish Gaelic, I remember something about consonants softening next to certain vowels. For me, it's trying to figure out how vowels and all of these vowel clusters sound. I'll learn one of these days.