What part of the country are you in? How convenient is access for you? The St Kevin's Way route from Hollyfort over the gap down into Glendalough looks interesting.. stunning scenery at any time of year. I spend a lot of time walking in that part of the world. The Bray coastal route doesn't look so interesting, it follows the coast up around Dublin Bay, but it's mostly urban, I spend a lot of time walking the dog in that part of the world. Whichever route you choose, try to get a couple of stamps and make a point of getting them to date them (from the nearest post office, garda station, restaurant, bar etc). Not everyone at the pilgrims office is 100% familiar with the Celtic caminos, but once you can show the credencial along with the one from the Ingles (from A Coruña?) and explain your case, you'll be fine. Interesting to note, A Coruña is officially only 73km from SdC, so technically any of the documented walks of over 27km should be ok..
Go n-eirí leat!
To add, there is the wonderfully situated Glendalough International Hostel at the end of the Kevin's Way stage. It's a lovely place to stay. It was closed last year and most of this year, not sure of the current situation..