Phonetics
V. — Modified or Aspirated Consonants.
A pure consonant, as we have seen, is caused by a sudden burst of breath or of voice through a complete stop in the vocal passage. When, instead of the stop being complete, a small aperture is left, and the breath or voice forced through this aperture for a moment, we have an aspirated or modified consonant.
To realise this we shall take the syllable pá.. It is produced by a burst of breath followed by the vowelsound. If a very slight opening be left between the lips, and an attempt made to pronounce pá., the result will be fá or phá(faw). A similar experiment on the syllable ba will give bhá., a sound which approximates to vau or wau, according to the size of the aperture and the force with which the air escapes through it. When a slender vowel precedes or follows, bh is like a v sound, when a broad vowel, like a w sound.
If we try to pronounce the m of min without quite -closing the lips, we get something like veen, but with a distinct nasal ring in the v. This is the pronunciation of mhin. When the accompanying vowel sound is broad, the result is usually more like w.
If we try to pronounce te without allowing the tongue to quite touch the hard palate, the result is the (hye). The corresponding sonant d when aspirated gives the sound of simple y. Similarly, ceó and gé give soundslike hyo and yae, except that the hy and y are pronounced at the back of the mouth, between the tongue and soft palate.
The c of cú is an explosion of breath between the edge of the soft palate and the back of, the tongue. If a slight opening is left here, and breath blown , through it before the vowel is pronounced, the result is chú. G in gá is pronounced by an explosion of voice through the same position, and in ghá by blowing voice through a similar opening.
The effect of aspiration on t and d broad is exceptional. In the case of th the resistance that might be expected between the blade of the tongue and the teeth, or hard palate, is completely removed, leaving nothing but a naked h sound. In dh this resistance is also removed, but is replaced by a resistance at the back of the tongue, giving dh broad the same sound as gh broad. This cannot have been the original sound of dh. Indeed, the old sound is not yet lost. I have often heard it in such phrases as Dia 's Muire dhuit: although the gh sound appears to be more common. This original dh sound is not at all unlike the genuine English sound of th in though and thou. It is, however, fuller and broader. It is a fine sound, and should be preserved.
Two other consonants, s and f, are aspirated. They are not pure consonants; hence the effect of aspiration is to widen an opening that already exists. It does this to such an extent as to remove the special resistance altogether. As both s and f are breath letters, the natural result of aspiration would be to leave a simple h sound. This is always so in the case of sh. In the fase of fh, however, even this h sound is usually lost and the letter is silent.
Irish phonetics - Rev. M. O'Flanagan - 1904