My dd (Y1) is best friends with a child who is visually impaired (registered blind) who has a 1 to 1 TA. The TA is doing courses in learning Braille which is very important, depending on the level of visual impairment, do you know if this will be a requirement? The girls TA along with helping her learn to read and type Braille also helps her use a large magnifying machine to help her read and look at things in the class. The TA helps make sure the classroom is safe and helps the girl navigate the school, most of the time my dd or one of the other children will help her get around (they are all so used to it that they just don't think about the girl holding on the their jumper as they walk around) she also stays with the girl at break and lunch time play.
In the 'behind the scenes' sense the TA is involved with planning to ensure each lesson is accessible and planning the Braille teaching alongside the class teacher. She also at the start of the year put Braille on every sign in the class and on the timetable cards (in our school they have small cards on the board every morning with the days timetable, they say the name eg - Maths and a picture of a sum, the TA then adds Maths in Braille at the bottom).
I am sure there is lots the TA does that parents like me don't see, I only know the amount I do as my dd is best friends with the girl and the TA is a lovely lady I chat to often (although she doesn't talk about the girl obviously but we often chat while she is setting up for the day). It may also be worth noting that the TA also helps out with other children from time to time, normally in group work that the girl she works with is in, she has been a great help and support to my dd who has some very mild SEN in regards to speech and language.
I think the most important thing is around the Braille issue, is it required in this setting, if the eye sight deteriorates will it become necessary (dd's friend has limited sight and can read text with strong magnifier but is likely to lose what she has over time and something like conjunctivitis could take the last of her sight hence she is learning Braille, that also reminds me that the whole class has to be hyper aware of any eye infections and must not come into school if they even suspect it before they have seen a doctor, if could be a good idea to see if this is relevant to the child you would be helping and reading up on signs and symptoms), they wouldn't I don't think expect you to already know Braille, I would imagine there aren't many applicants out there that would but I know in our case the TA is doing courses that keep her learning just ahead of the girls, they have quite a camaraderie about it and it benefits the whole class, all the children do a little bit of Braille from time to time, it all becomes very inclusive and to be honest if you didn't know there was a visually impaired child in the class and you came in and watched the class function you wouldn't guess until you saw her sit at her special table with her Braille writer and magnifier, the girl is very happy and doing very well at school, the TA will move with her through the school as continuity in this case is very important, I know the school and the girls parents would be far from pleased if a TA wasn't committed to seeing out the girls primary years with her.
I hope this helps a bit, sorry for the length!