Day Trip From Dublin

It's a long day, but you can do a bus tour to the Giant's Causeway from Dublin.
I'm a fan of Newgrange.
Kilkenny is too far and not worth the time.
You can make a day trip to Belfast from Dublin. It's a whole other country.

You can reach several interesting seaside towns within 30 minutes on Dublin’s commuter rail system called “DART.” These include: Dalkey ( Dalkey is known as one of Ireland’s most scenic villages); nearby Killiney (not Kilkenny); Malahide and nearby Howth. Most of these towns have castles and you could easily visit two in a single day before returning to Dublin.

We took the train to Howth and did the Cliff Walk on a beautiful mid-September day. It was one of my favorite days of our 18 days in the Irelands. I imagine it could be a chilly walk in March, though.

There’s too many good options from Dublin. I’m working on my trip for May. These are the places I have on my list as day trips: Howth, Dalkey, Bray, Malahide Castle and gardens, and Kilkenny.

I think I’m also going to add the Wild Rovers tour to Belfast and Giants Causeway. That would be a long day though.

Howth would be my top choice, if you have decent weather. If the weather is less than ideal, i would take the 1.5 hour train to Kilkenny.

Good luck deciding!!

Sounds like you'd really benefit from perusing a guidebook or two to get some perspective on the various destinations you've listed ... which are all terrific choices.
Many here on the forum recommend Kilkenny as a favored day trip from Dublin - easily done since it's only about 1.5 hrs via the train from Heuston station. I think there's but service too but you'll have to Google it to sort that out.
I'm inclined to agree that combining Newgrange, Trim and the Boyne Valley (on an organized tour probably) would be the best use of your time. There's a lot of Irish history packed into a small area.
Early March might be a bit too early for Glendalough and the gardens of Powerscourt, but those two are always popular alternatives.

If the coastline is a priority then I would vote for Howth and Malahide. You could probably do both in one day. Take the Dart to Howth, take one of the five cliff walk hikes, https://www.theirishroadtrip.com/howth-cliff-walk/, get lunch in the village, then take a cab ride to Malahide (about 20 minutes), go to the castle (really a manor house), stroll the high street and have dinner at one of the many good seafood restaurants. Dart, bus or cab back to Dublin depending on where you are staying. With four people taking a taxi to and from any of these locations would not be an unreasonable expenditure. Maybe $35-40 each trip.