I remember the first time I heard an Irish accent. When I was a small kid, an Irish person was speaking on the news and I asked my parents, "What language is that guy speaking?".They laughed and told me he was speaking English. After that I tried listening really hard, but couldn't understand anything at first. After a few minutes, I'd figured the accent out and could understand everything.

Now I sometimes won't understand a sentence spoken with a strong accent if I'm not expecting it, but I generally adjust to the accent after a few seconds. I sometimes don't understand a word or confuse a word with another, but there's not too much of a problem.

It'll probably take you a bit longer to learn to understand Irish and Scottish (or any other unfamiliar English speaking accents) accents for the first time because you're older than I was and because you're not a native, but if you have no problem understanding standard British or American accents I don't think it'll take you more than a few hours of exposure.

I do think Scottish accents are harder to understand than Irish accents, but not by very much. To me an Irish accent is closer to a BBC accent than a Scottish accent and I think an Irish accent is a lot more musical, but a lot of the time it takes me a few minutes to tell them apart. If I meet someone from Scotland or Ireland I usually have to ask them "Are you from Scotland or Ireland? (or the much more subtle "Where are you from?")  to be able to tell, especially with women.

I wouldn't worry about the accent of the place where you're going. Unless they try speaking to you in a separate language like Gaelic, understanding their accent is just a matter of getting used to the slightly different sounds.

Whit ye oan aboot laddie? Kin ye no un'erstoan me? I dinnae ken why foreigners cannae un'erstoan us :P

But no it really depends on where you are in Scotland. Edinburgh and Inverness usually have a mild accent. Weirdly even though Inverness is far north they're often mistaken for English. The Glasgow accent I can see as being a problem.

Personally I find Irish difficult to understand but then I can't really make a comparison. 

In my opinion it depends on where you go, I'm from Edinburgh and people from other countries usually have no problem understanding me. I think it's the same thing in Ireland, in some areas the accent will be 'softer' and therefore easier to understand but in general you will get used to the flow of it after a while!

There is a person from London who can amazingly speak the English language in 24 different accents.  <a href="http://www.youtube.com/watch?v=dABo_DCIdpM">http://www.youtube.com/watch?v=dABo_DCIdpM</a>;

In the movie, Braveheart, Mel Gibson speaks with an authentic Scottish accent. 

"Scottish accent vs Irish accent"   <a href="http://www.youtube.com/watch?v=QHt6HsTvSY4">http://www.youtube.com/watch?v=QHt6HsTvSY4</a>;

You need to realise that wherever you go to study you will come in contact with a wide variety of accents. British and Irish universities are very cosmopolitain, and your fellow students will speak with accents from every corner of the globe. These are the people that you will spend most of your time with. And even when you venture out of the student environment, you will still meet people with many different accents. Wherever you are in the British Isles, it is likely that the girl serving you in the supermarket may be Polish or Slovak and your taxi driver may be Turkish, Syrian or Pakistani. So, basically, the 'local' accent - whether it's Irish, Scottish, or anything else - is really not a big issue. You'll soon get used to it.