Prosthetic f, backformation or eagal and feagal

Prosthetic sounds are nothing unusual in the… ah OK, right you are. A prosthetic sound is a "random" sound that gets stuck in front, the middle or at the end of a word to make it conform better with the sound rules of whichever language.

Many languages do this, such as Spanish. Spanish phonology (sound laws) has a rule saying that no word may begin with [sp] [st] or [sk]. However, there are words in Spanish which either have these initials historically or have them from loan words coming into Spanish. To get around this problem, Spanish phonology says "prefix [e]" … so Latin spīritus, scola, stabilis become espirito, escuela, and estable. Similarly Scandinavia, spot and Stockholm become Escandinavia, espot, and Estocolmo.

Backformation means that speakers of a language take a native or adopted word and re-analyse it, as it were. This often happens in loanwords or native words whose meaning has become opaque. Opaque, when a linguist uses the word, means that the meaning is not clear from just looking at the word. For example, you can look at the word drumstick and figure that it's a stick that's used with a drum. But no matter how hard you stare at a hot toddy, chances are you won't figure it comes from a Hindi word for a palm-tree.

Where was I? Ah, backformation. English, for example, adopted the French word for cherries, cerise from Latin ceresia, both mass nouns which means there is no singular form. But because English has a very prominent -s plural, English speakers "figured" that the s must obviously be a plural. So, if many of those little fruits are cherries, one must obviously be - a cherry. This is called backformation. Gaelic does that too, but because it has the phenomenon of lenition at the beginning of words, unlike English, it tends to create backformations at the beginning of words.

Now, Gaelic phonology does not forbid words beginning with a vowel (don't laugh, some languages have no words beginning with vowels!) but it does have something like prosthetic sounds. Most notably [f]. In the case of Gaelic this comes about when native speakers re-interpret words beginning with a vowel as "a word which has lenited X". There is more than just one backformation however. Let's have a quick look at what can happen:

[h] ⇨ [t]	because lenited [t] yields [h]
initial vowel ⇨ [f]	because lenited [f] is silent - and this is by far the most common one, and about the only one, still active in modern Gaelic
initial vowel ⇨ [t], [g] or [d]	this happens in words with a soft onset which is an almost [j] sound, as in English iota, which to Gaelic ears sounds like the word begins with gh th or dh

There are more, but those are the most common ones.

Examples to your heart's content, a lot of them are loanwords, except the ones beginning with f + vowel:

Source	Meaning	Gaelic	Irish
Latin iota	a bit	tiota	giota
Latin fundus	foundation	bonn	bonn
Norse hǫll	hall	talla	halla
Norse þopta	thwart	tabhta	tochta
Norse Þórmóðr	(name)	Tormod
Norse Þorketill	(name)	Torcall
Old Irish áinne	ring	fàinne	fáinne
Old Irish allas	sweat	fallas	allas
Old Irish ásaim	I grow	fàs	fás
Old Irish ecla	fear	(f)eagal	(f)eagla
Old Irish ilur	eagle	(f)iolaire	iolar
Old Irish osclaicim	I open	fosgail	oscail
Old Irish rádharc	vision, view	fradharc	radharc
Old Irish uacht	cold	fuachd	fuacht
Old Irish uath	hate	fuath	fuath
Old Irish urusa	easy	furasta	furasta
Scots haggis		taigeis	hagaois
Scots/English worsted		bursaid	mustairt
Scots/English hoe		tobha	n/a
Scots/English halberd		taileabart	halbard
Scots/English hatch (n.)		saidse	haiste
Scots/English heckle (n.)		seiceal	seiceal
Scots/English hogshead		tocasaid	oigiséid
Scots/English throng		trang "busy"	trang(láil)
Scots/English viscount		biceas "busy"	bíocúnta
Scots/English vice	(tool)	bidhis "busy"	bís
Scots/English vervain		bearbhain "busy"	beirbhéine

Let's talk about your sister

A particularly bizarre example of this is the Gaelic word for sister because it went through this several times:

1. Indo-European used swesōr.
2. Proto-Celtic used swesūr - Proto-Celtic, as in before Brythonic (Welsh 'n all that) and Goidelic (Gaelic 'n all that) split.
3. Old Irish used siur AND fiur. OK, this requires some explaining. In the days before Old Irish, the words for my, your and his already caused lenition. So far so good, but this meant that Proto-Celtic swesūr became hwesūr, just like leniting s today. But as the hw never sat well with the language (a bit like thing becoming fing in London) it soon turned into an f, but our, your (pl.), their didn't, and didn't lenite. So, by the time we arrived at Old Irish, unlenited our/your/their sister had become ar siur, far siur, a siur, but lenited my/your/his sister had become mo fiur, do fiur, a fiur. Yes, slightly confusing.
4. To get from Early Irish fiur/siur to the modern forms, we now have to go via the genitive forms for Gaelic (something which did not happen in Irish - don't sigh). So modern Irish just continues with the old roots using deirfiúr "sister" from deirbh fiur "true sister" and fiúr "kinswoman" (because two words are better than one…).
5. However, Scottish Gaelic grabs the old genitives, fethar/sethar, and muscles them into piuthar. The p appears, of course, because f is quite obviously lenited p. Now why didn't you think of that before? ☺

Anyway, to get back to the point, this is the reason why you will hear some Gaelic speakers pronounce certain words with f and some without. It's a process that's still in the middle of happening, so until the language decides on whether it will stick with eagal or adopt feagal, you can take your pick about which one you prefer. It makes no difference, either way, as both are "good Gaelic words".