That's temptingly close to the Beara peninsula. Would you be able to travel that far?

While in Killarney, be sure to swing down to Ross Castle and get the boat across to Innisfallen (only a 10 minute trip). Well worth the effort, and while it's not megalithic, it's a real gem.

Advise
Cairns at Loughcrew Co. Meath

Carrowmore Megalithic Cemetery Co Sligo
Carrowkeel Cemetery County Sligo
54.2484 N
8.5193 W
(Includes 14 Cairns)

Augnacliffe Portal Tomb & Cleenrah Portal-tomb 1 mile away
County Longford

The Burren area Co Clare, have some great sites and locations if u are interested
Clonmacnoise Monastic site Co Offaly, not megalith but a must visit if you are in the midlands
Cill Rialaig famine village Ballinskelligs Co Kerry, not megalith but mega visit:-)
Shanballyedmond court tomb Co Tipperary
52.40.45.6 N
08.13.46.7 W
Fourknocks Passage Tomb County Meath
53.35.56.09 N
06.19.31.07 W
Some beauties in the Dingle peninsula co Kerry

Dublin?
Carlow
Browneshill portal tomb Carlow 150 ton cap stone
Haroldstown portal tomb Carlow
Kilkenny
Knockroe Passage tomb, County Kilkenny (Leac an Scail)
52.25.54.01 N
07.23.59.09 W

Kilmogue Portal Tomb, County Kilkenny
52° 24' 11.27" N
7° 15' 39.1" W
Waterford
Gaulstown Portal Tomb Co Waterdord
Knockeen Portal Tomb Waterford
Matthewstown Passage Tomb near Tramore Waterford
Ballynamona Lower Court Tomb, Co Waterford
52.00.12.07 N
07.34.52.04 W
Ballyquin portal tomb Waterford
Cork City
Rostellan Portal Tomb near Saleen, Midleton County Cork
51.51.24.6 N
8.10.55.2 W
This gem is on tidal land and accessible from the seashore when the tide is well out
Labbacallee Wedge Tomb Co Cork (Largest wedge tomb in the country)
52.17416 N
8.33456 W
Glantane east wedge tomb North Cork
52.0038 N
09.04468 W
Glantane East N stone circle North Cork nearby
52.00439 N
09.04485 W

Drombeg Stone Circle, West Cork
Island Wedge tomb near Burnfort County Cork

Altar Wedge Tomb west of Schull in west Corkon the R592
1km west of Arderawinny Portal tomb
Easy to locate just off the main road

Gurranes three fingers, West Cork
1.2km NW of Castletownshend
51.53083 N
9.19057 W

Killarney
Lissyviggeen Stone Circle 3km east of Killarney

Limerick
Lough Gur Wedge tomb near Hollycross County Limerick
Grange Stone Circle lough Gur Co. Limerick nearby, its huge
Duntryleague Passage tomb Limerick
52.40711 N
08.32546 W

I could go on and on, if u want to see these, have about 1000 images of Megaliths on Facebook
Charles Coughlan
Enjoy your stay in Ireland Eardrum

Dublin - National Museum

Carlow - Brownstown Dolmen

Waterford - purchase Fourwinds book on Waterford - loads of stuff but require a bit of effort.

Cork City - the UCC has a permanent exhibtion of ogham stones and rock art which is nice

Killarney - there is a stone circle just outside the town - Lissyvigeen

Limerick - definitely go to Lough Gur - easy to get to from the city and a beautiful place.

And as an aside - have you booked carlow and kilkenny? Id recommend Galway or somewhere further west (maybe west cork or kerry)over these.

Bogman covered it all really there but I'd agree that Lough Gur is a must if you are going to Limerick...and Duntryleague.

You MUST visit West Cork :

As well as The Gurranes near Castletownsend already mentioned, this is a must with stunning views.

http://www.megalithic.co.uk/article.php?sid=85

Also you really have to visit the four West Cork peninsulas. They are spectacular. Beara as already mentioned is a must, but you really should treat yourselves to all of them .

http://homepage.eircom.net/~jearley/beara.htm

Also, check out Bantry ( wonderful Bay, vibes and food ), and for great sounds, the internationally renowned De Barras in Clonakilty. If you're lucky, you may bump into Roy Harper as well !

http://www.debarra.ie/

If you can, get one of the many boat trips out to the islands ie. Sherkin Island from Baltimore as well.

Congratulations to you both, and have a wonderful time!