Labhrás, you are right. It is dá mhéid +nominative. Sorry about that. dá mhéid allas. And right on the point about lú, rather than lú-de. Sorry about that too.

On other occasions, the surprising standing of the noun in the bunfhoirm, i.e. what is now the nominative-accusative of the noun, can be explained by reference to accusative usage, a use that is no longer morphologically distinct. For example, in an té is lag creideamh, O’Nolan argues [p213] that creideamh is the ‘accusative of respect or specification’, stating in respect of what the person is weak. This construction is usual with comparatives and superlatives:
106. Ní raibh duine... ba mhó áthas ’ná Niamh. [p213, unsourced, but from Niamh, p28.]
Similarly, time and space phrases commonly stand in the bunfhoirm, which can be analysed as the accusative of time and space. In d’imigh sé an cnoc suas (given unsourced on p215), an cnoc suas is a noun phrase of adverbial meaning standing in the accusative, although the accusative is morphologically identical to the nominative in modern Irish. An oiread san is, O’Nolan points out (p214), an old accusative of time, which is now applied to non-temporal relations too.
Yet the accusative case cannot be cited to explain all instances of non-declension. O’Nolan points out (p220) that proleptic a and dá are followed by the nominative absolute (and not the genitive). E.g. a luighead airgead, where airgead is in the nominative. Other examples: dá fheabhas rí; dá luighead é thu, etc (all three examples are given on p220 unsourced, but the construction is commonly found). Such detailed specification of where nominative and accusative usage is being employed may be tied to O’Nolan’s familiarity with Latin grammar; learners of modern Irish may find it easier to see parallels between ba mhó áthas and a luighead airgead, rather than regarding one as accusative and the other as nominative usage.
Finally, O’Nolan points out that phrases in apposition can be out of construction: an example is ba mhór an t-uathbhás é, an té a chífeadh é—an example for which no source is indicated on p220—where an té is in the nominative absolute (with no attempt to use the preposition le before it). Some sentences include whole phrases not governed by prepositions that are also out of construction:
107. Ar tháinig gach rí an líon a gealladh? [pp160, 220, from Táin Bó Cuailnge, p37.]
This sentence means “did each king come with the complement (of men) that was promised?” O’Nolan could also have pointed to Ua Laoghaire’s comments on the use of féachaint:
An Irish substantive is frequently used alone as a word expressing manner, time, occasion, etc., according to the nature of the substantive. For example: féachaint d’á dtug sé thar a ghualainn chonaic sé… Here one might expect a preposition before féachaint, le féachaint, or something of that sort. [Notes on Irish Words and Usages, p135.]