Learning two languages!
So far learning irish is a little harder than french.
I am probably pointing out the obvious, but I think the difficulty of a language probably depends heavily on the language background of the student. For one example, I speculate that Russian is a lot easier for a person knowing (only) Ukranian than for a person knowing (only) English.
I was very amused to take a progress test in Irish and score twice as high as my test in Italian (disclaimer: both scores quite low), because I am sure I can make far more out of an Italian text than an Irish text. I deduced that the progress tests are oriented toward the vocabulary and phrases that duoLingo teaches. I mean to say, if they gave me a newspaper article in Italian, and one in Irish, I would have no clue at all what the one in Irish was about. But I would likely have some concept of the contents of the Italian one, due to the vocabulary crossover--exactly as you (RyanSucco) say.
Irish spelling is actually very consistent with the pronunciation. The issue is that Irish pronunciation is much more particular than most languages, and the latin alphabet really isn't properly equipped to deal with it. Leabhar and Leabhair are actually distinctive when spoken (by a native Irish speaker...).
I don't know too much about it, but I would guess that it came with Christianity. My understanding is that there was little writing in Ireland before then (there was some Ogham stuff, though) and also that Ireland converted quite easily, so my guess is that it happened quickly and people weren't too opposed.
Of the languages I have attempted or currently attempting I have to say the Irish is the most difficult latin alphabet language I have tried. Granted I didn't stick with Dutch or Italian very long, but dropping those was more a reshuffling of priorities. (can't learn all the languages at the same time.) But for me I must say the word order and dropping words really throws me.
Like "ithium" is "I eat" not "eat" things of that nature.
The most difficult I would say is Arabic and the only reason is the alphabet change. Trying to learn a new system like that takes a lot of pre-study before you can get into words and sentences.