You did not make a mistake.
Erie and NI are both worth exploring.
IMHO 3 full days and night in Dublin would be more than enough.
DUBLIN:
If weather is good enjoy people watching in St Stephen’s Green. Grab a bench and enjoy a coffee. Same can be said for the nearby and usually quieter Iveagh Gardens.
Trinity Library/Book of Kells/the Long Room
Ages ago 2 friends and I were visiting Ireland and Northern Ireland. Attempting Europe on $5 a day travel. Book of Kells was far more exposed than it is now and not that well known. On our visit we were the only people in the room. Thus plenty of time to enjoy the brilliance of the masterpiece.
When we climb the stairs up to the Long Room my friend was lagged behind. At the top of the stairs het sister and I turned right to enter the Long Room. We froze. My friend half a stair case below asked
“ whats wrong?” “Nothing. Thought we’d wait for you.” She arrived at the top and we let her go in first. She jerked to a stop, turned and announced
“I want you to bury me here.”
She’s an avid reader and the Library Long Room is a site to behold! A favorite travel memory!!
Speaking of reading the independently owned Winding Stair bookshop is worth a peek.
Walk along the River Liffey path. Take your pics on Ha’Penny Bridge.
Visit Merrion Park and the Oscar Wilde statue.
The mummy’s in the unassuming St Michan’s church.
BELFAST:
Titanic Museum
Seafood chowder and/mussels at Mournes next door to Kelly’s Pub
Crumblin Gaol
Black Cab Tour
The Capital Building
Bobby Sands Mural
Weather is always a challenge in the Winter months in the Northern Hemisphere but so what. There is a reason its called the Emerald Isle.