SR22
Air Traffic Control Fully Certified
Air Traffic Control Time Certified In Pos 1 (yrs) 9
Workload
ATC Issue All Types; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter CFTT / CFIT
Air Traffic Control Issued New Clearance
Human Factors
Aircraft X was inbound to GPI from the southwest.  He was passing through a 9500 ft. MIA (Minimum IFR Altitude) area.  I inadvertently cleared him to 9000 ft.  I was on the tail end of a busy session where I had been working multiple GPI approach/departures in addition to activities on the east side of the sector including Aircraft Y that needed a reroute and numerous deviations.  I did not have a D-side until very shortly before the incident.Have more staffing; make MIA numbers larger.
ZLC Controller reported descending in error an aircraft below the MIA.