Before I respond to your question, I’d like to invite you to get in touch with me if you’re going to be in Dublin at any stage during your visit to Ireland. I am relatively free for the next few months and if you would like it, I’d be happy to meet you to talk about topics of mutual interest.
Now, to your question. Ireland is well-stocked with Neolithic remains (Newgrange being perhaps the best known), and castles (difficult not to trip over one on a ramble through the countryside). Unfortunately, the same cannot be said for Celtic monument. The Celts did not live in cities (almost all our cities, including Dublin, are Viking foundations) or, for the most part, build in stone, either domestically or monumentally. So there are few surviving Celtic remains where you could walk where Brehon judges carried out their trade; some hill forts (dún) are all I can think of. The Hill of Tara and Dun Aonghusa are the most prominent of these. The Brehon law was a social rather than a concrete institution and with the collapse of Gaelic Ireland in the early 17th century, that institution collapsed.
When I was a student of archaeology, my vade mecum was Peter Harbison’s Guide to the National Monuments of Ireland, which I found invaluable as I wandered around on my little motor bike. It’s probably still available in an updated edition; if so, it’s worth taking a look at.
I hope we get the chance to meet while you’re here.
With every good wish,