Does it differ from english?
As galaxyrocker said, not really. As Irish has evolved, its punctuation has grown more similar to English.
Why was this marked down? It's blatantly true. Irish punctuation up until the 18th century was very different to English, then in the 19th century began to approach the format of English. You had several punctuation marks in earlier Irish texts that don't exist in English.
Previously, instead of an séimhiú there was an apostrophe type mark. So, for verbs and such, as far as I know, it'd be written as "c'onaic mé" instead of "chonaic mé". Being a modern Irish speaker, I don't know how the old language works as much as other people might. My father says the reason for the changes is easier reading, to accommodate foreign learners as well as the fact that typewriters didn't have everything needed for the old language.
Not sure how accurate this is, just what's been passed down through my family.
It was a dot rather than an apostrophe, I think it was changed for the sake of typewriters, adding a dot meant reversing the printing head which slowed down typing. The other thing is the dot meant having to use a special script, with the t looking more like a tau so the rising stroke wouldn't occlude the dot.
Another difference was the use of the tironian et rather than an ampersand.
These are really just differences in orthography, I wonder if there were differences in punctuation too, for example, did Irish use the semicolon for example, or the em dash?