post trip possibilities from Dublin

Yes, you can go to Wales ( either ferryboat or fly)
The ferry landing is on the west coast of Wales and you could book a short tour to someplace or area specific in Wales. You might look into a travel guide or online for suggested short tours. You don't say how long you will have in Wales..... but I think a trip there is worth the effort.
There are several reviews on tripadvisor about tours in Wales and particularly Fishguard (the ferry port area) as a pick up point.
Happy Travels

If you want to see Wales, in addition to the ferry, you can fly from Dublin to Cardiff (for southern Wales) or Liverpool (near northern Wales).

In addition to Wales, you could fly from Dublin to Liverpool, England on Ryanair (about an hour) starting at $37. Liverpool has some excellent museums, plus lots of Beatles sites. The harbor is interesting and scenic with good restaurants and night life. The city buses are good.

You might consider taking the bus from Dublin to Galway City, which has a different vibe than Dublin and the other places on the tour. While in Galway, you can take bus and ferry to Inishmore Island, which is amazing and very different from the other stops on the 8 day tour. You can do all of this with public transportation.

These are such good suggestions. Lots to investigate. Thank you all for sharing your expertise.

Take the the train to Wexford from Dublin. It is the most scenic railway line in Ireland. It skirts Dublin Bay and continues down along the Wicklow coastline, before coming inland through the Wicklow mountains and continuing down to Wexford.

Wexford town is absolutely great. Originally a Viking settlement, it was a thriving medieval town. It still has the tight-knit streets and it has a real character of its own. Also, it's a great place to shop, with lots of independent shops on Main Street, as opposed to the cookie-cutter UK chain stores that line the high streets of the bigger cities. It will be completely different to anything you see on the Heart of Ireland tour.

You can book tickets online at www.irishrail.ie and they are cheapest when booked in advance.

With just one or two days you can't go very far or you will spend most of your time getting there and then back again to fly home. My suggestion would be to try for three extra days and take the train to Belfast. From there you could do day a day trip up to the Giant's Causeway, then spend a day around Belfast and fly home from here. I don't think the tour you're doing covers Galway, either, another place easy to get to by train and lots of day tours you can do from there. But, you will have to return to Dublin to fly home. Your other option is to just stay in Dublin and do a day tour to some place you haven't already covered. Is Glendalough covered on the 8 day tour?

Anita--you are correct, Glendalough is not covered on the trip. Been to Galway. It would be easier to stay in Ireland and do some more exploration.
Marcus--Wexford looks intriguing. Sounds like there's a good garden to visit. Have already visited the botanical garden in Dublin. Would the sights in Wexford be accessible by public transportation or by walking from the train station? Or would taxis be a better way to go?

Wexford town is completely walkable. If you fancy a longer walk, you might like to take a taxi out to Curracloe beach and go for a walk (or a swim) there. There is a forest walk circuit at Raven Point which brings you in a loop out through the forest and back along the beach. The beach is pristine and has miles and miles of sand. It's also where they filmed the D-Day landing scenes for Saving Private Ryan!