Roberta,
Deehommed is in Drumgooland parish. Moneyslane is the local electoral area. So for electoral purposes, a number of townlands are grouped together. In this case the name of that electoral district is Moneyslane but it doesn’t alter the parish. There also happens to be a townland of Moneyslane within Moneyslane electoral district.
There’s about 19 townlands within the civil parish of Drumgooland. (RC parish of Drumgooland Lower whose records start in 1832). You can find all the townlands, and their parishes etc on this site:
https://thecore.com/seanruad/
You’ll have spotted that the spelling of many townlands varies all the time. The site uses what might be called the standardized version, as found in Griffiths Valuation. The Griffiths clerks in the 1840s and 1850s tried to standardize the spelling, for obvious administrative reasons. They weren’t always entirely successful and many townlands are still spelled in several different ways to this day.
These links give a little background and a map of the parish showing where all the townlands were in relation to each other:
http://freepages.rootsweb.com/~rosdavies/genealogy/PLACENAMES/DaDro.htm
http://freepages.rootsweb.com/~rosdavies/genealogy/MAPS/DrumgoolandTownlands.htm
In case you haven’t used the site before there are dozens of Gilmores (spelling varies) from Deehommed listed here, eg the burial of what is probably the Arthur in the 1790s lease, on 5th May 1837 aged 77.
http://freepages.rootsweb.com/~rosdavies/genealogy/SURNAMES/G/GilmoreGilmerAJ.htm
There were obviously several large families of Gilmores in the townland in the 1800s. Good luck with piecing them all together.
Here’s a link to the deeds on the Familysearch site. The deeds are indexed under Grantor’s name and also Land index (townland). You should search under the Land Index (townland) because you don’t know who the grantor is going to be nor does the index tell you which county the deed was in.
https://www.familysearch.org/search/catalog/185720?availability=Family%20History%20Library
If you find a likely deed in the index, you note the deed number and then look the actual document up in the main batch of records. You will need some patience. It’s fiddly. The indexes have been in use for 300 years (they start in 1709) and the bottom of many pages are grubby from thousands of hands turning the pages over. The handwriting is hard to read, and the deed numbers require a lot of peering to be sure you have the correct reference. Then reading the actual deed can be challenging too. However the documents tend to be formulaic and you get used to the standard working after a while. There's a lot of repetition, reciting the same names and terms and conditions, so you can skip some of it. Good luck anyway.
Explaining the general background, when someone entered into an agreement eg a lease, mortgage, a marriage settlement etc, each party got a copy of the document. But there was always a risk that they might be lost or disputed in the future, especially a very long lease for example. Records were always being lost as a result of flooding or fires in those days. So as a form of insurance the Registry of Deeds was created and, for a fee, a copy of the document could be kept there. It was a transcript known as a memorial. It wasn’t obligatory to register these documents and many people didn’t thereby saving the fee. But many did, especially if a bank was involved. They like their security.
To save you some effort I looked at the indexes for 1739-1810 for Co Down. There’s only 1 mention of Decomed, and that’s on page 68. It’s blank! Whatever the clerk intended to enter in the book, 250 years ago, he didn’t. But you may have better luck with earlier or later years.