by all accounts, ogham was a crude alphabet, and lost favor when better ones became available.. as for reading it.. i suspect it was't as hard as it looks.. we read text, and come across Y2K, or MMII, and and know exactly what is meant.

i suspect that literate societies didn't have a clear alphabet, to keep writen knowledge a bit of secret.. a kind of jargon (think of software manuals for computers!)