Appendix:Irish prepositions

Historically, the Irish declined prepositions developed from the merger of the independent preposition and the possessive pronoun. There is also a set of undeclined prepositions.

Simple prepositions govern the dative case (with some exceptions), while derived prepositions govern the genitive.

Prepositions may trigger initial mutations on proceeding words.

Preposition lists and case governance
In Modern Irish, simple prepositions nearly always govern the dative, aka prepositional, with a few exceptions governing the accusative. Derived prepositions govern the genitive. In the lists below, prepositional governance is assumed unless otherwise indicated.

The declined prepositions are:  ', ', ', ', ', ', ', ', ', ',a, p ', ', ó/ua, ', ', ', '. 

The undeclined prepositions are:  ', 'a, , 'a, go/go dtía, ', ', ', a. 

One-word prepositions derived from substantives, all governing the genitive and all undeclined, include:  'g, 'g, 'g, 'g, g. 

The preposition g is ultimately derived from a verb, governs the genitive, and is declined using the personal forms of chuig.

All two-word prepositional phrases govern the genitive. Most are formed as combinations of a preposition plus a substantive, e.g., ', ',  etc.

Declension table

 * The preposition  = +  is declined as ar.
 * Three prepositions ending in vowels (le, fara, trí (sg only )) use the 3rd sgm form before the article, e.g. leis an....
 * Prepositions ending in vowels (de, do, faoi, i, le, ó, trí) undergo contractions with other parts of speech, in particular the article. These forms are summarised in the Wiktionary template.
 * Some contractions with the article, especially those of i, retain the s of Proto-Celtic :
 * i + an &rarr; insan > sa(n)
 * i + na &rarr; insna > sna
 * dialectic variants:
 * de + na &rarr; desna
 * do + na &rarr; dosna.

Third person suffixes
The third person forms of Old Irish simple prepositions are case dependent (accusative or prepositional). The singular masculine forms are irregular, but the singular feminine, and the plural forms are tractable:

In modern Irish, only fara has + e in the 3rd sgf, although it is not derived from an Old Irish preposition, but rather from ferr (the comparative of maith). All others have standardised to + i.

In the 3rd pl, only de and do have + ibh. All others have standardised to + u. The 3rd pl of le (leo) derives directly from an Old Irish variant.

Indefinite
The simple prepositions may trigger initial mutations on proceeding words. The paradigm in Modern Irish is very similar to Old Irish (cf. Etymology below), with a few exceptions, notably:
 * the "mixed mutation" prepositions: idir, ar, thar, gan
 * those derived with significant changes: as, chuig, roimh
 * amhail

Definite
In Old Irish, the article  triggers case-dependent mutations. In the singular, the accusative triggers eclipsis, while the prepositional triggers lenition.

As prepositional grammar evolved in Modern Irish, the dialects diverged. Ulster Irish favoured the prepositional lenition, while the others, the accusative eclipsis.

In summary, for prepositions governing the dative, with the singular article:
 * Ulster Irish: lenition
 * Other dialects: eclipsis, except:
 * ,, : lenition (Munster)
 * ,, : lenition

Notes
 * lenition is subject to the dntls rule
 * lenition tends to trigger t-prothesis on s, especially for feminine nouns, e.g., sa tsráid
 * eclipsis tends not to occur on d/t, e.g., ag an doras

Etymology table
Other prepositions:
 * < do
 * < de/do + ar
 * : re-purposed archaic present subjunctive of

Other Old Irish prepositions:
 * L, p
 * : antonym of '; whence ' etc.