。
We found out it was difficult to get into that disco.
Literal
We [topic-は] that disco [to-に] enter [nominalizer-の-は] difficult [quotative-と] understood.
The pattern [clause]+と+わかる is 'to come to understand that [clause]' — と is the quotative complementizer marking the embedded proposition. 入るのは ('entering') nominalizes 入る. 難しい is the predicate of the embedded clause. The whole sentence chains an observation (entering is hard) with its discovery (we found this out).