The straightforward answer is "we don't know, and we're not going to, and we don't even know if that was really a thing".

But let me expand on that a bit. First, there are no written records from pre-Christian Ireland. Ogham inscriptions probably started in about the 1st century BCE, but they're almost completely limited to names and maybe some markings indicating land ownership. Christianity - arriving in the early 5th century - brought the first real written records with it, and Christianity was for obvious reasons disinclined to set down much about pre-Christian beliefs. While it's possible we'll discover more early medieval texts which carry some information about daily life, there simply wasn't writing before that.

Second, and largely because of this, we don't know enough about pre-Christian belief in Ireland to know if druids were a real thing at all. There are Roman accounts of them, certainly, but the Romans wrote some fairly mad stuff about the people they conquered, and they never really made it to Ireland. Most of the beliefs that are out there about druids at present come from the Romantic writings of the 19th century, overlapping largely with the Victorian period.

Even if druids were a real thing, it is much more likely that they were more like (or just were) the bards and judges of the Brehon law period (Early Medieval) than the priests of Christianity or spellcasters of D&D. Bards had a principal function of being keepers of information - law, in particular - and memorised long passages of what would in a more literate culture have been set down in writing. It's pretty clear from early writings that the overlap of the people called druids and those who were engaged in the profession of law was near total.

So to strictly answer your question, the rituals a pre-Christian druid would be called upon to perform would be things like "suing the neighbour for his cattle trampling the crops", "establishing the exact worth in number of cows of a cat who can hunt mice and purr", and "reciting particular stories by rote so that the next generation can memorise them".

If you're interested in druids as they actually were, rather than this Romantic/Victorian idea, I recommend Blood and Mistletoe: The History of the Druids in Britain, by Ronald Hutton.