The best documents would be birth certificates for each generation, but there was a massive fire at the Dublin records office in 1922, which destroyed a lot of genological records from before then. If you have any information about where in Ireland your great grandparents were from, you may be able to find local records however. Things like parish registers and birth records for sone denominations were stored outside Dublin, so you may be able to find them, although it’ll probably mean going there, or hireing to go there, as most of those records haven’t been digitised.
The best documents would be birth certificates for each generation, but there was a massive fire at the Dublin records office in 1922, which destroyed a lot of genological records from before then. If you have any information about where in Ireland your great grandparents were from, you may be able to find local records however. Things like parish registers and birth records for sone denominations were stored outside Dublin, so you may be able to find them, although it’ll probably mean going there, or hireing to go there, as most of those records haven’t been digitised.