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I was just wondering how common it is for applicants to have experience publishing with law journals prior to applying to law school - my Master's thesis (in an IR program) was selected for publication by a law school journal. Will this give me a boost during the admissions process?

It's a great soft, but it's still just a soft.
I had/have very good softs (possibly better, but I wont make that judgement) and it barely affected my cycle (in terms of numbers)

That's an awesome soft factor IMHO. Personally, if I'm an adcomm, I'm impressed. It seems like the kind of thing that can get you over the edge if you are a borderline applicant. Numbers are still paramount though.

I got part of my PhD thesis published in a top inter-disciplinary law journal, and I think it helped my cycle. I over-performed my numbers, and a couple of my acceptance letters included personal notes mentioning my research, so I think that's a good part of the reason why.

It probably depends to some degree on which law school journal it is.