Not come across, I can find no info for "lease renewable forever" on
https://www.prai.ie/ just reference to Leases for Lives Renewable forever.
Utilising google all can come up with is reference to perpetually renewable leases in England.
"A perpetually renewable lease is a lease which contains a contractual right to renew on the same terms, including the right to renew, therefore the tenant will have an infinite right to renew the lease. By operation of the Law of Property Act 1922 a perpetually renewable lease becomes automatically converted into a 2000 year lease and will be registered as such at the Land Registry."
https://www.myerson.co.uk/news-insights-and-events/the-danger-of-mistakenly-granting-a-perpetually-renewable-leaseHere is the section of the 1922 Act,
https://www.legislation.gov.uk/ukpga/Geo5/12-13/16/schedule/FIFTEENTH/enacted (and leases of lives became under that 1922 & 1925 Act into 90 years after death of original leasee)
But neither apply to Ireland.
Looking at the OS pages again both refer to college land. Think was on the right lines above with perpetual renewable for period of years.
See Enfranchisement Fee Farm Grants
http://mcmahonsolicitors.ie/redundant-legal-interests/ "Leases for lives renewable forever were perpetual interests, which equity protected by allowing renewals… Such interest developed from the 18th Century to some extent in order to circumvent the penal laws which limited the property rights which could be granted to a person who was not a member of the established church."
Also the entire paragraph from this page preview from 'Property and Trust Law in Ireland' by Una Woods
https://books.google.co.uk/books?id=9o2WDwAAQBAJ&pg=PT28&source=gbs_toc_r&cad=3#v=onepage&q&f=false which starts
A Fee Farm Grant is a conveyance of a fee simple subject to payment of a perpetual rent by the grantee and his successors in title to the grantor and his successors in title.
Conversion Fee Farm Grants came about through legislation designed to avoid the need for perpetually renewable leases for lives or leases for years.
and page 47 (pdf page 8 ) of
https://bahs.org.uk/AGHR/ARTICLES/61_1_3_solar.pdfThe more relevant sort of subletting involved ‘middlemen’, individuals who farmed little of the land that they held under lease, instead letting it out in smaller parcels to subtenants. In the seventeenth and early eighteenth century some Irish landowners, particularly those who had received recent grants of confiscated land, took the easy option of letting it out in blocks of hundreds and even thousands of acres. When they did so on leases in perpetuity or renewable forever, they effectively alienated the property to these large tenants. But many of the leases to middlemen were of shorter duration: in the eighteenth century leases for three lives were commonly given to Protestants, whilst the Penal Laws restricted the length of leases given to Catholics to 31 years. From the late eighteenth century landowners started eliminating the middlemen when they had the opportunity. If middlemen paid particularly low rents, either because they were favoured individuals or because they were taking over the effective management of the estate from the landowner,