I would strongly recommend to any American to visit France, specifically Caen (le Musée d'Embarquement), the museum made out of a German bunker in Boulogne-sur-Mer, Omaha Beach, and the American Cemetery in Colleville-sur-Mer. It is only in these places can one come to a full realization of how the world teetered on the brink in the 30's and 40's and how many people were slaughtered in the process.
I'm not at all crazy about London, but Westminster Abbey and the Tower Bridge are okay. Go to the top of St Paul's to the whispering gallery in the dome. The rest, meh. (and the food, my god, the food, horrific). If you want to say you've been to England, go somewhere more interesting like Bath, Oxford, Salisbury (near Stonehenge), and/or Penzance.
A day trip to Paris would be worth it just to experience the Chunnel (never have). Once there, one of those bus tours that take you around the city would be your best bet, because seeing the monuments themselves is the thrill. Standing in line to tour them is less so. To be truthful, Paris does not feature any great food to speak of, so go to a French fast-food like Pomme de Pain (sort of like Subway, but with REALLY cold beer).
Brussels is an okay city, very livable, and definitely where to go if you want great food, anywhere you want. It doesn't have to be the high-end frou-frou joints. Order moules-frites (mussels and French fries). Anywhere. If you go to the Grand Place, go into the Roy d'Espagne for a beer and to pet the taxidermied horse.
As for Spain, I really hate Madrid (but you will probably have to change planes there if you decide to go a different Spanish city). You will also have to go through European customs if flying from London because of Brexit. (yeah, a real pain in the culo). Barcelona is a great city, but it deserves several days, as does Seville, my absolute favorite. I LOVED that city, and I've been to a ton of them around the world.
I do not recommend Amsterdam. It is the smelly, hairy armpit of Europe.
I guess if I were you, I would just try to enjoy Dublin and some day trips to other parts of Ireland (European trains are great) to explore the area, rather than try to cram in other countries in a short space of time.