Well, shoot, but I think you've hit on my dissertation! The hard part is actually proving that captives from Ireland might actually have ended up as slaves in the Islamic world. Only then can we answer questions about what happened to any who might've ended up there. Like most questions about the early middle ages, it's useful to consider both texts and archaeology.

Texts actually suggest that slaves taken in raids in Ireland might've moved east toward the Baltic, not south toward Spain. The Life of St Findan talks about a wealthy young Irishman whose sister was captured in a raid sometime near the beginning of the Viking Age, around 800. Findan went to ransom her, but he got kidnapped for his troubles. He was sold through a chain of traders, before he finally ended up on a ship bound for the North Sea. He escaped on Orkney, at the north end of Britain, which suggests he was heading toward Norway or Denmark.

Around the 860s, commerce in the North Sea and Baltic was booming, and a handful of missionaries went up to preach in the emerging urban centers. The Life of Ansgar and the Life of Rimbert both talk about how these bishops were almost always in the midst of slave markets, doing their best to redeem Christians but focusing especially on those who had been trained in Latin and had likely been monks, nuns, or priests.

Further east, the 800s was a time when geography was popular among Arabic authors, and all agreed that the Islamic caliphate was accumulating a bumper crop of slaves sold to them by the Viking Rus. If you connect all these stories together, it looks like there was a lot of regional slave trading, both by viking raiders as well as among local inhabitants. But some captives ended up in major commodity markets, where they might be rapidly shipped down into the Islamic world.

Archaeological evidence also supports this impression. Slaves are almost impossible to identify through the archaeological record—you can't tell if a person was chained because s/he was a slave or a criminal, for example—but the exchange networks that made the slave trade are archaeologically visible. Again, these point to a booming trade with the Middle East, as well as continuous trade with Frankish merchants, but little if any evidence points to direct contacts between Scandinavian vikings and Spain.

So what happened to the slaves who arrived in Muslim-controlled territory? Their stories were probably remarkably diverse. Women may have ended up as concubines, or perhaps working in vast textile workshops. Boys could have been castrated and sold as eunuchs, or they could have been left in tact and sent to the mines. If they were older, they may have been sent to work on the massive construction and irrigation projects of the new Abbasid dynasty in Iraq. Some may have been integrated into the army, as servants loyal only to the caliph. There would probably have been very few men shipped south, since raiders usually culled any men who could potentially resist their captors.

In the long term, Islamic law was actually remarkably beneficial for slaves. Slavery was an accepted fact of life in most medieval societies, and all slaves suffered abuse and exploitation. But Islamic law placed restrictions on the abuse a pious master could give, and it offered incentives for freeing slaves and supporting them as your clients. So concubines might become wives, eunuchs trusted stewards, slave-soldiers prestigious generals, and field-hands adopted sons. It's hard to say just how well they integrated with the population, since modern Islamic states prohibit excavating cemeteries, and Spanish archaeology generally lacks the funding to answer these complicated questions of genetics and isotope analysis.