Double tax relief on irish rental income

i have a rental property in ireland and pay taxes both here and in ireland but i have never claimed double taxation relief previously.

with CRA, can i claim for the years i've not claimed previously in current year return, or do i need to amend previous years CRA returns?

also, for my 2016 CRA return, do i claim for tax owed to ireland for 2016 liability, or the amount paid IN 2016, ie 2015's liability that was paid in 2016?

help much appreciated.