I want to say a "stately monastery in which thousands of books are kept". Is it "mainistir maorga ina choimeádtar... or coimeádtar ..."? And is this the case for all saorbhriathar verbs?GRMA

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{Is geal leis an bhfiach dubh a ghearrcach féin}

Saorbriathar in the title should be saorbhriathar

Mainistir is fem.
Ina - this is the supposedly standard spelling, but the pronunciation is 'na.
The indirect relative takes eclipsis.

Mainistir mhaorga 'na gcimeádathar...

In good Irish (ie traditional Munster Irish) the autonomous form of the verb is not lenited ever.

Leabhair a cimeádathar sa leabharlainn.

GGBC probably has a different rule.

This is traditionally correct:

tigh a glantar
tigh a glanfar
tigh do glanadh
tigh a glanfí
tigh a glantí

I think there is some evidence of lenition in the subjunctive:
nár ghlantar an tigh.