Cen fáth...is bhfuil used sometimes and not Is? I'm on mobile and can't read the notes readily. Can some good soul please break that down for a lad?
I've just had the same doubt as you, and I've checked a similar topic: https://forum.duolingo.com/comment/8293653/C%C3%A1-bhfuil-t%C3%BA
As far as I can understand, Cá is a contraction form of cén áit a, so it already includes the particle a used instead of an in interrogative questions when there's a relative aspect.
I hope it is useful for you!