Kathleen,
First possibility is to consider the lease may not still exist. They did not all survive. I assume you have searched under townland + the tenant’s surname. If that has not brought anything up in the e-catalogue then they probably don’t have the estate copy. The tenants will have had a separate copy but that is much less likely to have survived.
A second possibility is that the lease was registered in the Registry of Deeds. Not all were. There was a fee to pay and some didn’t bother. The purpose of registering was to ensure a copy existed if the originals were lost. But as I say not all bothered. Those records are on Familysearch. There are 2 indexes. One by grantor and the other by townland. Use the townland index. Search by time period, county, barony and townland. If you find a deed mentioned write down the reference number which, typically might be 172,38765.153, You then look the actual deed up in the same set of records. It’s a fiddly business till you get the hang of it. Dr Roulston’s excellent book on researching Irish farm records talks you through it all.
If your family were tenants in the 1780s or 1790s, do check carefully that span of years on Familysearch. Many of the leases then were 3 lives leases which named 3 folk. Could be father and son for example with their ages. The lease would last typically 31 years or till all the names were dead. So a gamble for the landlord but secure income too and security for the tenant. Sometimes you’ll see that one of the lives was royalty, say the Prince of Wales. That was based on the fact they usually lived long lives. They weren’t involved in the lease, just a part of the timing process. Dr Roulston’s book explains it all.