an arts degree in any subject shows that you have academic ability, self-discipline, ability to work towards long-term goals, time management skills, etc etc. All the above plus, if you do it the OU route, in my opinion. History is also a teaching subject, if you want to go in for that.
Doing a Masters is often less to do with the subject you studied, as you can often do conversion courses if you want to do a Masters in a very different subject. But conversion courses aren't always necessary. Even if you wanted to do your Masters in Irish history, you should be able to do so without problems though you might need to do a bit of catch-up for yourself. Also, Masters' courses tend to be quite specialised, so your undergraduate work is nothing but a basis as you will be learning new stuff about a very small area and also generally learning how to do research for your thesis/dissertation.
Talk to lecturers in the university history depts yourself, they'll often have a quick word of useful advice. And I believe the OU has a very good career guidence service which you mihgt try.