Translation:We left in the morning and we went to Dublin.
This is a good question; Baile Átha Cliath doesn't take the article elsewhere. Gramadach na Gaeilge mentioned some other cases where you don't use the article, e.g. with pronouns: go dtí é. Can't find anything that explains this, though.
Doing a search on Google (in English, admittedly) gives 10x as many results for "go Baile Átha Cliath" than "go dtí Baile Átha Cliath", although the former might be used in fixed constructions like "fáilte go Baile Átha Cliath". So...I dunno?
Using go dtí with a city without an article can be found in the Irish language Bible, e.g.
Labhraíodh briathar an Tiarna le Ióna, mac Aimití: “Éirigh! Gread leat go dtí cathair mhór Nínivé agus fógair oracal ina haghaidh, mar tá mallaitheacht a háitritheoirí tar éis éirí aníos os mo chomhair.”
from the beginning of the book of Jonah.
I just had this translation, and Duolingo did accept "go Baile Átha Cliath."