If you look on the Irish boards (eumom.ie, rollercoaster.ie, weddingsonline.ie) you will see that this is a pretty emotive topic in Ireland. It tends to get the same sort of heated debate that vaccinations posts get on mumsnet!
Your sister will get good care irrespective of what choice she makes. However, I think SP care is a bit more pleasant than public care - slightly shorter queueing times, a named consultant (although she would see midwives on most vists), smaller wards, higher staff to patient ratio (I think), that sort of thing*. If her deliveries are straightforward and she does not need extra care, and will be out of hospital within 2 days of delivery these things are less important. If she ends up with a longer stay these can make a big difference. Also, as roisinanna said, cutbacks are having an impact on maternity services and sadly, this is usually more evident in public care.
My own view is that if the SP fees are not too much of a stretch for your sister, and this is her first baby, I would go semi-private. If she has had previous, uncomplicated pregnancies and deliveries, public is probably fine.
*will caveat the above by saying that I don't have direct experience of the above as went private on my children - so I could be completely wrong.