First off, your insane.
Dublin. Hmm, for a planning geek I'd recommend starting in the far east, around the the new docklands area, take a tram ride to the museum, (don't go in it not very good really), travel back east and cross at Mathew Bridge, walk around the Viking Area. Go through Temple Bar, (a pint here will force you to re-mortgage), go to the lovely old market on Great George Street, go east and take in the regeneration along the way, Grafton Street, and down to Nassau St, east to Merrion Sq, get the DART at Pearse Station north to Connely station which is where you started. That's a days worth easily. It is compulsorily to go out and drink in the evening.
London. Jeez where to start! Erm, wow… okay whistle stop tour. Goto to Canary Warf station and look around, tallest buildings in the UK are there. Take the DLR under the thames to Greenwich and walk up to the observatory for the views. Go to Greenwich market. Take the DLR back to tower Gateway and go ooh ahh at the last bit of the original roman city wall followed by looking at the tower of London and
the Bridge. Erm, go west into the city and gaze in wonder at the Barbican. Goto St Pauls and cross the wobbly bridge to the Tate modern gallery (its mostly free). Walk along the South Bank to Borough Market(London Bridge Station). Or don't bother and go west along the southbank to Westminster bridge via the London Eye. Cross the river to Westminster and see Big Ben. Walk through the city via Piccadilly and Covent Garden… erm… yeah anyway, plan it well and get a map, and you might get away with it.Go see St Pancras as the gateway to Europe using rail.Around Aldgate and Brick Lane is interesting as its the boundary between the semi-autonomous 'City of London' and the nieghbouring London Borough, so on one side of street is glass skyscrapers and the other typical english terrace housing.
Paris. Pretty much ditto … A lot of the Paris Metro is above ground (the orange line and the dark blue one I think) good views. Go up to Sacré Couer as the views are great:
Get to La Defense, go and see the catacombs. Montpannesse… West Bank, latin quarter. Further out St Denis is a suburb worth going to as it's the bastion of the French communist party, has a massive TGV station and the Stade du france is there.
Munich is much more walkable and compact. I only went there as a proper tourist, its very nice. I've writen too much and must go do some work.