Mark. There comes a point in Irish genealogy where everyone hits a brick wall because of issues with lack of records, and unfortunately it is fairly recent, 1800-1820s for most. Many people have gotten to the brick wall point that you are at, and have either given up on Irish genealogy or began YDNA testing. It's has become popular especially for those of Irish descent because it opens up more avenues of research. Unlike Autosomal DNA which fades away after a few generations YDNA will tell you who you are related to for thousands of years as well so you can see where your family branched off from other families over the centuries.
An extra benefit of YDNA testing is that you can connect to people that have a connection to a certain area or a certain person already and can join projects to further your Genealogy research.
I myself have tested the BigY at FTDNA, and am trying to find more Gillen/Gillan connections. Through autosomal DNA I have matches in county Mayo, Sligo, Donegal, Tyrone, and Antrim. As you have said in a post your Gillens may be of Scottish descent which is something I am trying to prove as well, YDNA testing can help. Reach out if you have questions. - Bill Gillen