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 (By similarity) (PubMed:11355701). Linked to its role in pre-mRNA processing mechanism, may also participate in retrovirus replication and have an antiviral cell-intrinsic defense function (By similarity). Belongs to the lariat debranching enzyme family.