I love History.
Okay.
First, you say why it's valid. Just list everything about it that you find valid, this bit's not hard.
The real marks come when you add in what could be disputed within the statement. Basically, you've got to write how it wasn't the "creation and maintenance of Northern Ireland within the UK". Although you may completely agree with this statement this bit isn't about your views yet. If it's too hard then nitpick - go all literal on them. Even if there are other minute factors then add these in and expand if necessary. It doesn't matter if the good points completely outweigh the bad, this is what the question is trying to get you to do.
Now compare both - Still not about your opinion yet. Just talk about how the positive and negative factors effected eachother and how they compare.
Now talk about your view on the matter - how you think that it is almost completely valid.
Now a conclusion to sum up everything.
Sorry that I don't know much about Northern Ireland, but there's how I would answer the question if I knew all the info'.