Madrid or Brussels or Dublin!?

Brussels? ewh. Madrid? mmm.

Madrid - 8 out of 10.
Brussels - 3 out of 10.
Dublin - 5 out of 10.

Maybe I'm just sleep deprived, but I cannot, for the life of me, follow your proposed itinerary. You're flying to Madrid, but the flight to Barcelona is shorter so maybe you will cut Madrid and go to Brussels or Dublin? Why does Barcelona have anything to do with Brussels or Dublin? You're flying Athens to London, but could fly to Dublin instead? When are you going to Athens? How long is this trip, anyway?

In order of preference for me: 1. Madrid
2. Dublin
3. Brussels (in truth, I am not the least bit interested in Brussels)

I have been to both Madrid and Dublin multiple times (will be going back to Madrid next May).

@Nancy

The flight from London to Barca is shorter/cheaper then London to Madrid. Because of this I am thinking of just flying to Barca and cutting Madrid.

I am then looking at where I should spend my days I now have free since I cut Madrid.

I am equally lost with Nancy. Is this still part of your 70 day backpacking or another trip? I am not a big fan of Brussels either. There are more interesting areas around Brussels - Bruges, Antwerp, a little further - Delft, The Hague. From what I think is your question - skip Brussels. Madrid is far more interesting than Dublin.

Why were you going to Madrid in the first place? If you had a good reason, is saving a "bit" on money and time worth skipping a place you thought you wanted to visit for 3 nights?

Belgium (Bruges/Ghent, not Brussels) could be a 3 night stop between Paris and Amsterdam without undue additional travel. From Athens, is it the same price and time to Dublin? How would you get to London?

Were you even planning on going to Barcelona in the first place, or only to take a shorter flight to somewhere? I am in agreement with Chani - if you wanted to go to Madrid so much that you were planning three days there, why would you ditch it in favor of a "cheaper" trip to a city you didn't intend to visit (Barcelona) so that you could go on to somewhere else (Brussels or Dublin)?

If the plan for your 70 day trip remains similar to the one first proposed, perhaps consider chasing the warmth of spring, starting in the south and finishing in the north.
1) Fly in to Athens for 7 nights in Greece
2) Fly to Madrid for 10 nights in Spain
3) Fly (or train) from Barcelona to Nice
4) Remainder by train (avg 3 nights each) - Nice - Cinque Terre - Sorrento - Rome - Florence - Venice - Munich - Vienna - Budapest - Krakow - Prague - Berlin - Amsterdam - Paris
5) Eurostar to London & UK for 10 nights
5) Fly out of London

I'm not a fan of Madrid. It's a new city by European standards and doesn't pack a lot of history. El Prado is outstanding but you don't need three days to see it. In April, I'd look at a couple of options.

If during Semana Santa, take a side trip to Valladolid to view the non-touristy festivities there (or catch a fast train to Zaragosa).

If during Seville's April Fair, check that out. It's a cultural experience in its own right.

If neither, I'd focus on nearby cities. Toledo is great. Segovia and Avila are also good visits and not far from Madrid. In the other direction Aranjuez is another good choice. A little further out is Cuenca.

Or you could go a completely different direction and visit Pamplona (plus castle in Olite) and San Sebastian to the north, or Barcelona to the east (by fast train or air hop).

Dublin has a lot of sights but Ireland is at it's best in small towns.

Brussels is a pretty town with great architecture. You probably don't need to spend long to see it, maybe a full day (two nights).

Brad is not a Madrid fan, but I am. While he says it's a "new" city by European standards, it has been in existence since the 1560s, and the capital of Spain since 1606 (not exactly new). It has lots of history, modern goverment buildings, beautiful parks, markets, huge art collections, and great food. Other than that, the suggestions of seeing Toledo and Segovia are good ones, too.

Like Nancy, I also like Madrid, and wonder what people saw or did when they describe it as a 'new' city.
Not old like Rome or Athens, but it does have some great historical sites ...
http://www.patrimonionacional.es/real-sitio/monasterios/6095
http://www.patrimonionacional.es/real-sitio/monasterios/6144

... Madrid a "new" city, hum.... when was the first proper city established in the US?.... just askin'...

1565 St. Augustine; define "proper". Some of our cities are definitely improper.