I'm pretty sure this is a bug but then again I'm no expert and I've been unable to find many people who've experienced it. Basically when I applied a mirror to an object and moved the object the mirrored object moved in the same direction. Normally the mirror would move in the opposite direction i.e moving object up z axis would cause the mirror to move down the z axis instead of up as the original object.
This all started when I joined a circle to an object which was being mirrored. The circle was joined but the mirror modifier was automatically removed. From this point on no matter if I reloaded the startup file, reverted back to default settings or rebooted the PC the mirrored object would always move in the same direction hence why I think it's a bug rather than a setting I've mistakenly applied.
I fixed this unexpectedly by adding an object, adding the mirror modifier then applying it. After this I tried adding another mirror modifier and it worked as originally intended. To make sure it wasn't a fluke I restarted blender added a mirror modifier to a new object and it's working as expected.