References to slavery and the enslaved come up often enough in legal tracts and the literary body, but it's never been clear exactly what their place was. Looking at the numbers of slave each king is owed according to this tract: https://celt.ucc.ie/published/T102900/index.html under "The Rights of the King of Tara" the numbers aren't all that large. And based on what I know about the social fabric and economy of early medieval Ireland already it seems like holding someone into total bondage was a display of great wealth; otherwise, why wouldn't you ransom them for livestock or something, which are literally valued hire than slaves for bartering purposes IIRC, and owning more of which gives you the chance of entering into higher social classes. Slavery-as-status-symbol tracks with that, but I've never come across any detailed research on the subject. Not that I've looked that hard, wouldn't be here otherwise.
Slaves would have performed the most menial agricultural labour depending on their gender - a male slave called a mug would have tended to livestock (in the case of Saint Patrick, for instance) and chopped wood while a female slave called a cumal would have ground oats in a quern and kneaded dough. Our knowledge of the sorts of labour performed by slaves in early medieval Ireland is mostly indirect, coming almost entirely from passages in legal texts. The bit about male slaves chopping wood, for instance, is cited in the Corpus Iuris Hibernici where it's mentioned that neither the slave or his master are at fault if he injures or kills somebody walking by while performing his labour.
Also, the text that you cited may not even be talking about actual slaves in every context. By the historical period a cumal had been reduced into an abstract value, usually payable in silver or gold rather than actual human chattel. Slavery was still widely practiced in Ireland but without a wider semantic context of where the term cumal appears, it's often safe to assume that a legal text is talking about an abstract value to be paid in precious metals or cattle than an actual female slave.
edit:
Actually, this separation between a cumal as an abstract unit of value and actual female slaves is reflected in the language of the Middle Irish original text: the Lebor na gCert does indeed stipulate the giving of slaves to various kings, but makes a distinction between a cumal (likely to be paid in gold or silver) and mná (lit. "women", the implication that they are slaves comes from their association with the other gendered term mogaid - the dative plural of a variant spelling for mug).