Sarah,
Sadly the 1891 census was completely destroyed during WW1. None of it has survived.
A foundling is a baby that was abandoned. (Perhaps a young single mother unable to look after it). Often left in the church porch where it would soon be found, hence “foundling.” The church would take it in, name and baptise it, and then – until formal adoption started in 1928 – they’d usually get someone in the local community to in effect adopt it, though there probably wouldn’t be any paperwork.
Both William & Annie gave their address at marriage as Lisbane, in the parish of Kilmood. Here they are still there in 1901:
http://www.census.nationalarchives.ie/pages/1901/Down/Tullynakill/Lisbane/
The valuation revision records (on the PRONI website) list William in a house on plot 17b in the townland. In 1901 there were about 44 houses in Lisbane. Population was 194.
Here’s a link to a site with a little information and some photos of Lisbane.
https://townlandsofulster.com/2016/10/10/lisbane/