"The seal eats an apple."
Translation:Itheann an rón úll.
There 2 major problems with your sentence. an séala is the kind of "seal" that you put on an envelope or a bottle or a box - those seals don't eat apples.
The other problem is that both Irish and English have separate forms for the simple present and the continuous present. "the seal eats"/itheann an rón and "the seal is eating"/Tá an rón ag ithe do not mean the same thing, and can't be swapped.
úll would also need to be úill after ag ithe