Before modern times and the English conquest, Ireland was never a unified country. It consisted of several individual kingdoms spread across the island. So the title "Royal Sites of Ireland" is a bit misleading. These are not the royal sites of the kingdom of Ireland, but of several of these Irish regional kingdoms.

During my Ireland trip I managed to visit both Cashel and Tara. Both are situated on hills overlooking the surrounding plains. As such, they offer a natural defensive positions and were natural choices to set up a castle and seat of government.

Tara is the older site, at least with regards to the remains. It consists of earthen mounds and forms, partially dating back to neolithic period. The Mound of the Hostages is the most significant structure. It's a passage grave and similar to nearby Newgrange. The site also houses an 11/12th century church.

While there is no evidence for an Irish high king ruling over the whole island, Tara was named as the ancient seat of the High King in the 11th century book Lebor Gabála Érenn. It has kept the reputation ever since. Personally, I am wondering if the local rulers didn't just make up the myth to bestow extra importance on their kingship.

Cashel is the younger site with most remains from after 11th century. The upside is that you have a tangible site to visit. Cashel covers the whole range of what you would associate as Irish: It's a castle, has a high cross and a ruined limestone cathedral. And nearby you find even more monasteries in ruin.

OUV

If I had only been to Cashel, I would probably concur with Ian's assessment to not inscribe. Don't get me wrong: I think it's a great site to get an introduction to Ireland. But it doesn't have enough OUV on its own. But Tara is unique and would provide some background on Ireland's past and development. Also taking into considering that Ireland's list as of 2019 is way too short (2), I would favor inscription.

Getting There

Tara is a simple local bus ride from Dublin. They will drop you off at the main road and it's a short walk up the hill. The bus continues to Kells, one of the Early Monastic Sites of Ireland, so you can (and should) combine the visits.

Cashel is a 2h bus ride from Dublin. The castle/abbey/ruin is smack in the center of town, so hard to miss. If you plan to travel to west coast, it seems a natural stop.