Tá ceist agam, a dhoaine léannta.

In expressions such as "i ndiaidh ama" and "i rith an ama," why is am inflected to read ama?

Because the first part is treated as a noun and the next element is in the genitive.

Can be called 'derived prepositions'

GRMA, a Bhearn. Is this commonly observed by native speakers?

Assuming they were using the genitive, the yes the second element would be inflected.

Maybe other people know (particularly among the young) if this is still the case. In set phrases I'd imagine everyone uses the genitive anyway

quote:

Is this commonly observed by native speakers?

In recent years a German author has had a series of bestsellers dealing in a jovial manner with the decline of the genitive case in the German language. ("Der Dativ ist dem Genitiv sein Tod.")

Of course, the question those trying to preserve the genitive in their respective languages will eventually have to face up to is: Are we being possessive?

The genitive is not often used in normal speech if there is some kind of qualifier with an indefinite noun:

'I ndiaidh tamaill', 'I lár páirce' etc

But 'I ndiaidh tamall maith' 'I lár páirc mhór'

The Coiste Téarmaíochta suggest recognising this tendency in the written language. See their article on www.acmhainn.ie/tearmai/seimhiu.doc

Má ghabhann aidiacht, nó ainmfhocal sa ghinideach i gcáil aidiachta, le hainmfhocal éiginnte, moltar an t-ainmfhocal agus an aidiacht a fhágáil gan infhilleadh i ndiaidh réamhfhocail chomhshuite nó réamhfhocail a leanann an ginideach é de ghnáth:
i ndiaidh breac mór millteach, ar feadh tamall mór, i lár garraí mór, le haghaidh doras gloine, le linn stoirm mhór, i gcaitheamh lá breá, de bharr cogadh fíochmhar, tar éis geimhreadh fada, i dtaobh báid bheaga

Yeah, "i rith an ama" is kind of a set phrase so everybody would put "am" in the genitive. But for the phrases that are not "set ones", maybe they wouldn't use the genitive, especially younger speakers.