There is no doubt that law school is challenging for couples, but it's definitely not a "relationship killer." Both people just have to keep their perspective, try to communicate, and try to make the best of a frustrating situation. Just a few things from my experience:
1) Make sure that the two of you are on the same page before it begins. This includes a lot of considerations when starting something as overwhelming as law school. You may want to think about things like when you will try to make time for each other (one day a week? A little bit each night?), where/how he expects to do most of his studying (Early in the morning? Late nights at the school?), where/how he plans to spend his summers (Study abroad? Move to a different city for an SA position?), etc. Some of this stuff may seem really trivial, but it can be surprising how quickly the little "oh, I thought you would come home at night to do your studying" moments can add up when the stress piles on.
2) It's only temporary. Remember that law school goes in four month sprints for three years. Between semesters, you can usually relax and reconnect, and when it's all done and he's passed the bar, it's over. Try not to get too narrowly focused when November rolls around, he's in the library all day, and it seems like you barely know him. The semester always ends and he will be back.
3) Make sure it's only temporary. This kind of goes with #1, but is more related to post law school. If you find that the busy schedules and nights away are extremely stressful, you may want to make sure that your SO isn't aiming for a big firm, big money job. Make sure that you're on the same page about where this is all going, and discuss whether your SO is planning on becoming an 80 hr/wk slave to the law for life. If so, consider whether this is the life that you want as well, because that, unlike law school, does not usually end.
4) Take an interest. Remember that your SO is going into an immersive program, in which he is expected to eat, sleep and breathe the law. For awhile, it will be all he wants to talk about, and every conversation will somehow relate back to it. First, remember that this is also temporary and is part of the process of joining this little cult. Second, when this is going on, take a little interest and ask some questions. He will be happy to work through his understanding of the material and you will get a window into his world. After that, kindly remind him to shut the hell up and listen while you talk about your day :-)
5) Remember, as hard and frustrating as it is for you, it's no picnic for him either. This does not mean that he is justified in ignoring you, or taking his stress out on you, but it does mean that you need to cut him a little bit of slack if he's not always there 100%. Just try to remember that he will be under a mountain of stress, and that he would much rather be spending time with you than stuck in the library with his nose in his books. That stress is exponentially larger when you know that there is a significant other at home growing more irritated by the minute. You don't have to feel bad for asking for some of his time, but when he is doing his best and there is just no time left in the day, try to be understanding.
Just remember that law school is a big exercise in artificially induced stress and contrived competition. There are times when it will be difficult for both of you, but eventually life will move on and law school will be a speck in the rear view. Spend as much time together as you can, and enjoy the ride!