Will have 2 days at the start and one day at the end of a trip there in June...

Any must -do's  or things only locals know to enjoy to get out of the tourist spots?

20 views, no replies?  C'mon lads....

Guinness factory and town bus tours.
Spend the rest of the time in Temple Bar.

That's all I remember of Dublin.

I stayed in the City Center. Lots of good pubs. Gunniess tour is ok. I did a lot of drinking. Was fun to just walk pub to pub and see the area. Too bad, I'm going back May 5th.

Seriously?  You post in the United Kingdom hometown forum and ask about another Country. It's like I post in your hometown forum and ask about visiting Calgary

Been there twice.  I really like the town.  Buy the Hop on/off bus tour.  Worth the money to hear the stories if you get a great driver.  
Hop on/off bus  there are two companies a Green color and Red color schemed Red vs Green.   Do mostly the same route if I remember correctly.
Along the route   Route Map
Kilmainham Gaol museum   I thought this was a very interesting look at the Uprising times.
Guiness tour and bar  I don't drink beer, so never went in.
Jameson tour and bar/tasting  One of my favorite stops, greatly improved from 2015 to 2018 visits.  Prices went up as well for tours though.
Temple Bar area, it is not just a single bar, the whole section of town is called Temple Bar.
Trinity College
National Museum of Ireland - Archaeology    I really liked this , free, alot of Viking artifacts.
St. Patricks Cathedral
Christ Church
and a ton of other stops.