"Aon bhràthair beag."
Translation:One little brother.
It's in the tips now if you're on the website. There's a little ancillary in that aon lenites everything that dà does except for words beginning with d, t, and s. My best guess for why not those ones is euphony. Because it tends to happen for other cases where words would usually be lenited following a word ending in n (sgian dubh for example, where sgian - knife - is feminine and dubh would normally lenite, but doesn't because of the n at the end. I suspect they're carefully keeping those examples out of the early lessons, enough to remember as is.)