Cleaves the 2'-5' phosphodiester linkage at the branch point of excised lariat intron RNA and converts them into linear molecules that can be subsequently degraded, thereby facilitating ribonucleotide turnover. Linked to its role in pre-mRNA processing mechanism, may also participate in retrovirus replication and have an antiviral cell-intrinsic defense function. Belongs to the lariat debranching enzyme family.