Locating a property on a Griffiths map today is really just down to careful map reading and sometimes knocking on a few doors to check you have the right location. You can follow through to see who the occupants were after the Primary VAluation, by using the revaluation records. They go up to 1929 and are in the Valuation Office in Dublin. Be aware that not all the population were listed in Griffiths. In general it was only people with land that were listed and only heads of household at that. So servants, people in many towns, people with property of too low a value to be listed etc are not there. So make sure that your ancestors were of a profession likely to have had land eg a farmer before concluding "this must be it.".
Statutory birth registration started in Ireland in 1864 so you should be able to find a birth certificate for Patrick born in May 1864. If so, that should give useful information on where the parents lived and what the father's occupation was.
Regarding a researcher in the Killybegs area, you mould try the local library to see if they know of anyone able to help. (I would e-mail them now, rather than just approaching them when you get there).