Trip to Ireland for a wedding, late May 2025

How wonderful you are able to do this. Because you can't do it "all," sometimes that is quite freeing--you can do one or two things well. It does not take long to traverse the country, so you can stop in Dublin a few days, drive to the wedding area and see cliffs en route or as a day trip. As long as you don't overdo it, this is all perfectly reasonable!
I would definitely try to fly in Dublin and out Shannon if you keep Dublin on the agenda, and I'd only pick up the car when departing Dublin.
I might pass on the Blarney Stone given the drive time, but if you enjoy drives and your child sleeps well in the car, that might be an excursion you'd enjoy.

Blarney is way out of your way. I'm not sure who considers that a major sight anymore. In recommending where to go/what to see, it always helps to know what folks' interests are. If you are museum people, you'll want to go to Dublin, which has all the major museums. If you're nature people. you definitely want to see the Cliffs of Moher--or maybe the Dingle Peninsula or the Ring of Kerry, both of which are fairly near to where you are going for the wedding. Once you decide what you want to see and do, then you can decide which airport to use. I don't know how easy it is to get flights into Shannon anymore. OTOH, there are buses to every part of Ireland direct from Dublin Airport, in case you don't want to rent a car immediately after coming off a transatlantic flight. If you decide to go into Dublin, you can take a train to, say Limerick, and pick up your car there. Or Cork if you really want to see the Blarney Stone.

Want to see the Ring of Kerry ? Its a 2,5hrs drive from Glin Castle to Parknasilla Hotel near Sneem on the RoK

They do have holiday homes and lodges for families on their grounds

Holiday Homes can be booked via dreamireland website

Lodges and Hotel rooms via Hotel website (see above)

Drive from Glin Castle to Killarney 1,5hrs and see the town do a boattrip on the lake see Torc Waterfall and the Nationalpark

Continue on the Ring of Kerry to Sneem to Parknasilla and stay for some nights its a wonderful quiet place

From Sneem you can do daytrips with Kerry Experience Tours along the RoK ,to Dingle or Beara

Don't underestimate driving times in Ireland. Driving is SLOW and it will take a lot longer to cover the distance than nyou might expect. With 'only' 7 days to play with don't be too enthusiastic and also remember to allow time to recover from jet lag. If flying into Dublin, you won't need a car to hit the high lights of Dublin (and do remember to think about the 3 month old when deciding what you want to see and do... ) Allow 2-3 nights for Dublin, then pick up the hire car and head west.

Glin Castle is surrounded by some lovely scenery. Cliffs of Moher are easily reached from it, especially if you use the Tarbert Killimer ferry rather than slogging round through Limerick. The Burren with its limestone scenery is completely different to the rest of Ireland and you could easily spend a couple of days exploring this. Alternatively spend a day exploring the area immediately to the south - towards Tralee. This part of Ireland is negelected by the tourtists but has some very pretty scenery and small towns like Listowel. The coast is also pretty with sandy beaches and cliffs (Bromore for example - they may not be as dramatic as Moher but there won't be the crowds). There are places with wonderful names like Ballybunion and Ballyduff...

Blarney Stone - this is going to be a couple of hours drive each way from Glin Castle through OK scenery. Ask yourselves if this is a good use of time. (Kissing the Blarney stone must be one of the biggest tourist cons ever....)

Consider visiting the picturesque Dingle Peninsula. It's only about a 1 hour, 30 minute drive from Glin Castle to Dingle Town, the main city of the Dingle Peninsula. The Dingle Peninsula is gorgeous and a good place for hiking, bicycling and sightseeing.

In 2010, I hiked the 100-mile-long Dingle Way trail (https://www.dingleway.com/). The trail is good for day hiking.

The Celtic, Norman and Christian sites on Dingle are pretty amazing. They include the pre-St. Patrick beehive huts (https://en.wikipedia.org/wiki/Cloch%C3%A1n) built by early Christians, spooky Minard Castle (https://www.megalithicireland.com/Minard%20Castle.html), Gallarus Oratory (http://www.gallarusoratory.ie/), St. Brendan's House (memorial of explorer who reportedly beat Columbus and possibly Lief Erikson to America), Kilmalkedar Church (https://en.wikipedia.org/wiki/Kilmalkedar), South Pole Inn (home and pub of South Pole explorer Tom Crean, https://en.wikipedia.org/wiki/Tom_Crean_(explorer)), dozens of others.