First, these are LRCAT (lower case) not ircat
With regards to what you can keep and what you can delete, we can't tell you because we don't know what is in these catalogs. You have to look in each catalog (you can open them by using File->Open Catalog in Lightroom) and see what is in them and decide if you need them.
The ones that say Previews.LRDATA (lower case) are not catalogs, these are previews. If you don't want to keep the catalog of the same name, you can also delete the previews associated with that catalog.
In the list of files you give I can only see ONE catalog file- "2015-2017.LRCAT"
2015 Previews.irdata-5/12/16 A PREVIEW folder.(can be deleted)
2015-2016 Previews.irdata-3/6/17 A PREVIEW folder.(can be deleted)
20105-2016.ircat.lock-3/6/17 A "Lock" file that remained when a catalog was incorrectly shut-down. (can be deleted)
2015-2017 Previews.irdata-Today A PREVIEW folder. (Keep)
2015-2017.ircat-Today *The only CATALOG that I see. (Keep)
2015.ircat.loch-5/12/16 A "Lock" file that remained when a catalog was incorrectly shut-down.(can be deleted)
These are the only two files you need ON THIS VOLUME. You have not shown us any other Volumes to know what other Catalogs you have..
I would like to add that this again is a good argument for having only ONE catalog. Then you never have to ask the question which one is my current catalog, and you never have to ask if you can discard other ones.
Thanks. Used to have one catalog but split them up to try to get a performance boost. Did't really happen. Now I'm thinking you are correct. One catalog is the way to go housed on my internal drive. Since my previews are the largest part can I put them on an external drive with my .lrcat on my internal drive? How do I go about combining the 3 catalogs into one single catalog?
I believe there is a way to have previews on an external drive and catalog on internal. The method is called "symbolic link" but you'll have to search for the details, because I have never done it.
To combine catalogs, open one catalog that you want to be the "master catalog", select File->Import from Another Catalog, and then select one of the other catalogs. Repeat.