Agree with the analysis that two days will allow a decent overview of Dublin but will feel squeezed in Edinburgh. A hop on/off bus in each city is worthwhile. Although perhaps an unfair comparison, measuring Dublin Castle versus Edinburgh Castle illustrates the issue between the two cities: Dublin Castle can be done in two hours while Edinburgh Castle could take much longer. And the views of the city and surrounding countryside from Edinburgh Castle are worth the price while Dublin Castle is in the middle of Dublin but without elevation--in fact, going to the rooftop bar at the Guinness Brewery is the best view of Dublin and should be done.
If you're of a literary or music bent, however, Dublin matches up against Edinburgh and might justify a longer stay of a day or two. I've had the luxury of multiple trips to both areas, but I definitely understand that if you didn't know whether you would return, you'd like to slip in no more than a couple of days in both cities.