While O'Connell Street is not exactly hidden, many visitors miss Dublin's famous market at Moore Street. Walk north on O'Connell Street and pass the G.P.O. (Post Office) on the left. Take the left turn following the G.P.O. which is Henry Street and Moore Street is shortly on the right. It is a riot of colour like most markets - unlike most markets you will hear the real Dublin cries and accent. Recommended. Look out for the little oranges, described as 'Mandrins'!
Another location often missed by visitors is the Catholic Pro-Cathedral, just off O'Connell Street on Marlborough Street (the Department of Education's classical lines are also worth a look here). Its classical columns and dome are worth seeing. There are two cathedrals in Dublin, both Protestant [Church of Ireland] - the Catholic church labelled its building in Marlborough Street believing that one day it would be granted tenure over St. Patrick's or Christ Church cathedrals. While you are in the area, you might live to visit Dublin's most famous (and probably oldest) department store, Clerys, on O'Connell Street and North Earl Street. O'Connell Street [formerly Sackville Street before independence] has seen many changes since the 1800s, but the cornerstones consisting of the General Post Office or the GPO [rebuilt after the War of Independence], Clerys and the Gresham Hotel [north end of the street, not far from the Parnell monument] still remain.