Thesis Issues Only I Could Have

I’m writing about the founding of the Irish Women Workers’ Union. My rule of thumb when writing about historical figures is to just use their last names after I have given their full name the first time I mention them. The problem is I have already mentioned both Jim larkin and Delia Larkin and now I need to mention them again, but I can’t just use Larkin. Is it cheating if I refer to them by their full names from here on out? Or would it be better to use J. Larkin or Big Jim for Jim Larkin and D. Larkin for Delia?