I've only been once and it was a decade ago, but I'll just share a few of my highlights from my 10 days there.
Dublin:
- My best advice from my experience is if you feel tempted to try out the big "touristy" pubs (Temple), go once, get your fill, and then try all of the smaller pubs and establishments you can. I noticed they each have their own character. I highly recommend you get to "The Brazen Head", dubbed the oldest pub in Dublin, for at least one pint.
- If you have a free evening and are looking for some good food and live entertainment, I had a lot of fun at Johnny Fox's Pub with my group. Live music and dancing performance, good food, etc. I believe you'd need a reservation for that bit but it was fun and it was up in the mountains, so off the beaten path. They have shuttles.
- St. Patrick's Cathedral has a really nice tour and tons of history for you to review, and if you have some time to kill between stops but don't feel like drinking, I recommend a walk around Trinity College and check out the library.
- If you're lucky enough to have a sunny, clear day in the city, that's when the Guinness tour is the most worth it for the almost 360 degree views overlooking the city at the end of the tour. Otherwise, I'd honestly skip that. It's cool, but there's other things that you could be doing.
Most of my journey outside of Dublin was in southern Ireland, but we did do Newgrange one day which was really neat. I'm not sure how much time you'll have on your way to Roscommon or on your way back to Dublin from Galway, but if you can stop, I highly recommend it. You might need a reservation for that as well.
Enjoy, it was easily one of the most enjoyable times of my life. Such a fun trip and country, so much history, natural beauty and phenomenal architecture all around you. Sláinte!