It is illegal to supply medicines by mail order, i.e. the supplier (and anyone who facilitates them by, for example, providing the office building from which they process orders) is committing an offence.
It is not strictly speaking illegal to get medicines by mail order, i.e. your friend would not be committing the offence although the supplier would.
HOWEVER, if her medicine is also controlled under the Misuse of Drugs Acts and Regulations (and if it's a drug that has street value, it probably is), then it's illegal for her to possess it unless she's received it on a valid prescription, and a prescription that's been written for the purpose of getting something by mail order is not considered to be legally valid. So she could be done for illegal possession, not specifically the fact she got it by mail order. [This only applies to controlled products, not all products.]
Also, would you be happy to get medicines from someone who knowingly breaks the law by supplying stuff illegally?
By the way, the monthly DPS limit is now €85, not €78.
But if she's spending, say, €70 per month on meds, she'd be well over the limit for getting tax back on this, especially considering her GP fees as well, and particularly if she (or her spouse if she's a dependent) is in the higher tax bracket.
Say €70 x 12 = €840 per year.
Substract the tax threshhold of €125 giving €715.
42% tax back on €715 = €300.30. (More if you throw in GP fees.)
Unlikely she'd make that much of a saving from internet purchase - if she did, I'd be worried about what she was receiving.
(I doubt she'd get tax back on internet purchase since it was from an illegal supplier.)