Yes it's Bratt - meaning mantle/cloak and it's lenited (consonant softened) because of the personal possessive pronoun before it "do" ("your") which changes the sound of the b in bratt to a v "Vratt" (spelt bhratt or bhrat in modern Irish, bratt in Older Irish - the sound of the lenition /v/ is there but it's not generally written on lenited b, g, d in Old Irish, only on c,p,t).
The translation is:
O Bridget, part* over my head
your white** veil to protect me
* the word "scar" is from the verb scaraid, meaning to separate, part, spread-out, unfold - so you can use any of these words in the translation. Brighid is being asked to cover the person making the prayer, with her mantle of protection.
*fionn = means variously: white, pale, bright, etc so again, use any of these words in translation. Fionn is the modern way of writing find or finn (as in Finn mac Cumhaill) with the above connotations of whiteness, shining, bright, etc.
The phrase "ós mo chionn" means "over my head":
ós is a preposition meaning "over", "above" and takes the dative case of noun following thereafter. Although the dative case is lost in modern Irish its form is preserved in such phrases.
"mo" is a possessive pronoun "my" which causes lenition (consonant softening) of the c of cionn, hence chionn.
Cionn or ciunn is the old dative case of cenn "head" (Modern Irish spelling "ceann"), a neuter o-stem that becomes a masculine-o-stem in Middle & Modern Irish, rather than the dental stem cinn for 'sin', 'fault' 'crime', 'transgression' etc., whose dative would be cinaid (or the short form cinn)
The approximate phonetic English pronunciation of the phrase as written above is as Abhaill said:
A Vreeda, scar ohse muh khyunn
do vratt fyunn dum anagul
Some notes on pronunciation:
1. when pronouncing ch as in chionn and gh as in Brighid, you soften the sound of the c and g by making the same kind of sound as in the name of the composer Bach or the Scots/Irish gaelic for lake: Loch. It's not a hard /k/ like "back" or "lock" but a soft /kh/ sound. Same with "gh" - the h showing the g is softened by lenition, so instead of a hard /G/ like in "garden" you soften it to something approximating the ch in Bach but with a slight g-sound instead of the c-.
2. I'm not sure if the fionn (white/pale) should be lenited after the lenited Bratt /vrat/. Adjectives follow the noun they qualify in number, case and gender and mutations such as lention or nasalisation apply depending on the gender (masc/fem or old Irish neuter) and case (nominative, accusative, genitive, dative in Old Irish or nominative and genitive in modern Irish) of the noun. The noun bratt is lenited following the possessive pronoun but I'm not sure if the mutation should also be applied to the adjective too. Normally masculine nouns such as brat and their qualifying adjectives, are lenited in the genitive case, but this is nominative, so I can't remember whether to apply the lention or not. Ah well....(if lenited it's do bhratt fhionn /duh vrat ee-un/, if not "do bhratt fionn" /duh vrat fyunn/ which I choose).
On the form of the invocation:
I'd actually put "A Bhrighid" (if using modern Irish spelling, or Old Irish "A Brigit") rather than "A Bhride" because the A is the "vocative particle" used when hailing or addressing someone directly ("O Bridget" in English) and the name of the person or thing should likewise be in the vocative case. Bríde is a more modern variant name for Brighid - it comes from the old genitive (possessive case) Brigte in Old Irish, nominative Brigit. When you are calling upon someone directly, the name (in old language) should be in the vocative case which is the same as the nominative, so Old Irish: A Brigit, which is written A Bhrighid in later and modern Irish; and both are pronounced "A Vrigh-yid" (the B changing to a V-sound because the vocative particle A lenites (softens) the B sound to a v-sound). In Middle Irish and modern Irish, some old genitive case forms of names are used for the nominative eg. Dannan for Danú, Bríde for Brigit/Brighid etc.
hope that's useful - maybe the grammatical explanations are a bit more than what was asked for but thought it might be helpful to know where some things come from.
The phrase as it's written in the Book of Leinster (LL 308a48) is:
A Brigit, scar os mo chind*
do bratt find dom anacul
*here the d on chind is a middle Irish hypercorrection. Cenn as far as I know never had a final d unlike the old form Find** which later becomes finn and in modern spelling "fionn" due to assimilation of the nd sound to nn.
Lenition is pronounced on the B in Brigit and Bratt (v-sounds both)but not shown in Old Irish writing convention.