Some men (mainly freeholders) had the right to vote in the 1800s. This was gradually extended at various points in the 19th century. 1918 saw the vote extended from the previously limited categories to all men over 21, and for the first time, to women over 30. In Northern Ireland 1928 saw the age for women reduced to 21 in line with men. (Not sure what the position was in the Republic)
Electoral rolls in Northern Ireland seem very hard to come by. One local electoral office I spoke to said they send the old rolls to PRONI in Belfast, but I have not been able to find them there. I don’t know where they are kept.