Sure, here's an ideal itinerary for one day in Dublin:

Morning: Start your day with a hearty Irish breakfast at a local cafe or restaurant. Some popular options include The Fumbally, Brother Hubbard, and The Woollen Mills.

After breakfast, head to Trinity College to see the Book of Kells, a beautifully illuminated manuscript from the 9th century. You can also take a guided tour of the college campus and learn about its history.

Next, visit St. Patrick's Cathedral, one of the most iconic landmarks in Dublin. This stunning cathedral is over 800 years old and has beautiful stained glass windows and intricate carvings.

Afternoon: For lunch, head to Temple Bar, a lively neighborhood filled with pubs, restaurants, and shops. Grab a pint of Guinness and some traditional Irish fare at The Temple Bar Pub or The Porterhouse.

After lunch, take a stroll through Phoenix Park, one of the largest urban parks in Europe. You can rent a bike or just walk around and enjoy the scenery.

Evening: In the evening, head to Grafton Street, a bustling shopping district with street performers and musicians. You can also check out the famous Molly Malone statue and browse the many shops and boutiques.

For dinner, try some modern Irish cuisine at The Winding Stair or Chapter One. After dinner, catch a show at the Abbey Theatre or head to a pub for some live music and a pint of Guinness.

Overall, this itinerary will give you a taste of Dublin's rich history, culture, and cuisine in just one day.