Gilwil,
For Roman Catholic baptisms, the child is normally baptised within a few days of the birth. The religious belief was that if the child died without being baptised it would end up in limbo. That concept is no longer valid but it was in the 1800s. So normally birth and baptism are very close together. That’s not the case with other denominations where a delay of 3 months or even some years was quite common and, as you say, a group of children may be baptised together. But this was presumably a Catholic baptism and it would be very unusual to delay in that faith.
The only Margaret Gallagher birth in 1896 registered in the Inver area was on 28th June 1896. She was Margaret Josephine Gallagher and her parents were Patrick Gallagher and Bridget Slevin. So that doesn’t look like the birth you are seeking.
I am not sure what your source is for the baptism date of 4th March 1896, but unless you have a baptismal certificate (which I assume you don’t) the date needs to be taken with caution. People didn’t celebrate birthdays in Ireland in the 1800s and rarely knew when they were born. If officialdom asked them for an age or a date of birth, they often made one up. So the information can often be out by quite a few years. Given that there isn’t a Margaret/Maggie Gallagher birth registered anywhere near March 1896, I’d suspect this might be one of those.
I am not sure the family I found in the 1901 census can be yours. I looked for them in 1911, and found that their Maggie Jane appears to have died. I can’t actually find a death certificate for her but the census says they had 9 children, 8 of whom were alive. 8 are there save for Maggie Jane so presumably she had died.
http://www.census.nationalarchives.ie/pages/1911/Donegal/Dunkineely/Drumnakilly/479470/
We really need your Margaret Jane’s parents names from her US marriage or death certificate to be sure of tracing the correct family. The 1901 census of Donegal has 8400 people named Gallagher/Gallacher and 331 of them were named Margaret or Maggie. So lots of scope for finding the wrong family.