Immediate thought, your Dad and his Siblings are hiding their knowledge of family events!
Alas Irish records are very sporadic, I am 25% Irish and have had great difficulty building my tree and I have been doing so for 15 years.
I would work through your highest cM matches, ignoring any on the Maternal side of your family.
Look at the shared matches, build trees of them and build them into the trees of others. This would be better happening in software where it is easy to combine trees when a MRCA between matches is found.
You could also take a y-DNA test with FTDNA and that will give you your Male to Male lineage. I did just that when my First Cousins all failed to take a DNA test, and found the person I thought was a direct ancestor to be correct.
DNA can be a waiting game so do be patient.
Meanwhile take time to learn about DNA, its inheritance and relationships ie First Cousins share in your case the same Grandfather and you and them are likely to share around 750cM with each other. If a Grandfather is unknown and if he then went on to have further children by a different woman then their Grandchildren would be of the same generation as you. They would be a 1/2 First Cousin and share about 400cM, do note cM for a relationship does cover quite a range.
As others have said uploading DNA to Gedmatch, myFTDNA, My Heritage may yield further results. These will only be useful with DNA knowledge so do study.
A 275cM match to me led to finding one of my Great Great Grandfather’s who was Italian, the match had a tree of him and his Dad and failed to respond to Messages. A 364cM match led to my yDNA test and that shows my Irish Great Great Grandfather to be who I thought him to be. So do not give up it can be as I have said a waiting game.
Hi, Sorry for not replying sooner, I have been unwell over the past two weeks and just stating to feel better again or I would have replied sooner.
One of my cousins has tried to find out about or grandfather and more about him and who his mothers family was. I don't think she got much farther than I have. I asked my uncle a while ago and he said if he can get the research my aunt and cousin were doing, he will pass it onto me, although I am not sure how long that will take.
I would work through your highest cM matches, ignoring any on the Maternal side of your family. - I have done that. Most of my closest matches are either on my paternal grandmothers side or my maternal side so I have ruled them out as I mentioned in my first post.
One match I have 2nd-3rd cousin (246cM) I am pretty sure could be related to my grandfather, and I have contacted her and we don't yet know how we are related.
When I checked related matches to her, it came up with a bunch of 4th-6th cousin matches to people that either have the surname McMullan/McMullen or have direct ancestors who share that surname so I assume that is a connection.
I have a 4th-6th cousin match who's direct ancestor (maiden name McMullen) was at the birth of my great-uncle, but tracing her family leads to a brick wall when her fathers parents marriage certificate doesn't mention the father for him, only his wife. I have started trees for people in the area the family lived with the McMullen or McMullan surname which might help.
Some of the DNA matches I do have with those surnames are in the USA or Canada and don't know where in Ireland their McMullen/McMullan ancestors came from, only Ireland or "County Down" in Ireland which doesn't exactly pinpoint anything as they would have came to the USA pre 1850s, before birth certificates were issued in Ireland in 1864.
I am sure they are is connected to my grandfather somehow as any that do have a location for their ancestors is in roughly a 30 mile radius of where my grandfather was born which is nowhere near my paternal grandmothers family or where my maternal family lived and if they did, I can find the connection.
I also don't have any close matches to anyone who either is or has McKnight/McNeight/McNight as a surname for a direct ancestor, only 5th-8th cousins so that does not really help.
Have you uploaded your DNA to any other sites? You may find useful matches at FamilyTreeDNA and MyHeritage.
I have on GEDMatch but not the other two. My mum and some of her relatives have on MyHeritage.