The Neutral Vowel of Irish

Irish has a neutral vowel used in unstressed syllables, much like the unstressed vowel in English lemon or April. The exact value it takes depends on the quality of the surrounding consonants, broad or slender.

Between broad consonants, or between a broad consonant and the end of a word, it's a schwa, much like the unstressed vowel in English but, only shorter:
 

agus	and
asal	a donkey
cumas	ability
focal	a word
bosca	a box
leaba	a bed

Between a slender consonant and a broad one it's an extra short e, as in English bet:
 

airgead	money
aiteas	strangeness
cuireann	puts
éifeacht	significance
Éireann	of Ireland (genitive)
taitneamh	shine

Between slender consonants, or between a broad one and a slender one, or between a slender one and the end of a word, it's an extra short i, as in English bit:
 

álainn	beautiful
cuirim	I put
focail	words
ligim	I allow
duine	a person
oíche	night

The spelling of this neutral vowel has been one of the bugbears of Irish orthography. It arose historically from different vowels in different words, and traditional spelling preserved the historical origin, just as in the English examples lemon and April above. The modern standard uses (e)a to spell it before a broad consonant and (a)i before a slender one (with the vowels in parentheses used where necessary as marks of broad or slender for the preceding consonant). But there are words even in the modern standard which preserve the historical spelling, such as agus "and", which by the new rule should be spelled agas, or words like acu "by them", rompu "before them", tríothu "through them", or words like duine in which the historical final -e is preserved in spelling.