The documentation is not too difficult to get. Both the UK and the Republic Records office sell copies of birth, marriage and death certificates for around £7 or the equivalent. You just need the complete set to prove your ancestry. Grandmother's birth, her wedding, your mother's/father's birth , their wedding, and Your birth certificate. All those can be ordered online.

I am not sure what is needed if people were unmarried, but otherwise it is not too onerous, especially if you already have the approximate dates.