Well the way I thought up Eilis is basically that she is a conductor, like metal for example, normally when she was unbound (the tattoo's) she wouldn't have problems calling forth and releasing magic. As much as metal can conduct heat. However too much heat and you know what happens.
 
The problem is that she is bound. They set a magical binding on her, which was put into her skin, very much like a tattoo, so that it would react instantly when she used magic.
Since it blocks the release, her magic can't really get out of her body. It strains and damages her body. 
She can't use her full powers/range and if she does use any form of magic she will feel terrible pain, naturally gradually to the amount she uses.

Currently Eilis experiences the pain of lashings, it comes and goes with the strength of magic she put into her efforts and since she does not want to die...you get my point. 
But I don't think mages would be too quickly exhausted, especially not trained ones. 

Also right back at ya [@TheMerlin]:

[url]https://www.youtube.com/watch?v=3tzE98pQH08[/url]

[url]https://www.youtube.com/watch?v=FCnGD6xv5ik[/url]