Depends on your interests really. I visit Dublin several times a year for work & these are some of thr things I've enjoyed"
Visit the crypt of St Michan's where Bram Stoker is buried. It's a v interesting tour
Have coffee & a cherry bun in Bewley's on Grafton St & see the beautiful Harry Clarke stained glass windows
Visit IMMA & enjoy the grounds
Visit the Tenament Museum 14 Henrietta St - A faded grandeur Georgian St
Visit the RHA gallery & enjoy a glass of wine in The Ely Wine Bar in beautiful Ely Place
Kilmainham Gaol - v interesting
GPO museum - v good on 1916 Rising
EPIC Museum on Emigration
Have a wander in St Stephen's Green
The Litttle Museum of Dublin
Do a literary pub crawl
Visit the James Joyce Centre & do a Walking tour of Joyce's Dublin
Trinity College / Book of Kells & Douglas Hyde Gallery
Have a cocktail in the tiny upstairs cocktail bar at Peruke & Periwig (book a table)
Visit Christchurch & then have traditional fish & chips from Dublin's oldest chipper Leo Burdocks around the corner
Dublin Castle & The Chester Beatty Musuem there
Catch a performance at The Olympia / The Gaiety / The Gate / The Abbey / The National.Concert Hall
Visit The National Museum / The National Gallery
I was in Dublin last month & the city centre was no worse than most other cities. I travel a lot with work. On this occasion I walked down O'Connell St in the morning & it was busy. Lots of people about. I was on Capel St & Grafton Street too. No problems at all.