Ultimately its up to the girl, and you should probably know what she would want, but i like things simple, so something like this:
randomly take a day off work and take her on a picnic or just for a drive then a walk in some area of natural beauty, maybe a nice but deserted beach like at killalee, or just some place with a nice view. Then without kneeling tell her i love her and i want to spend the rest of my life with her que ring[birthstone preferably, and hopefully not a diamond though i might concede that point if she really really wants it] and question.