Hobby chargers including PowerLab are not going to give a meaningful IR number unless it's just for comparison purposes. Charging and discharging shouldn't have significantly different readings for TRUE IR.
While this seems like a gimmicky unit, I have several, and while their correlation isn't fantastic (fuzzy at the 0.3mΩ level), they do provide meaningful IR measurements. Search for YR1030 or YR1035. They tend to run $40-60. Personally, I prefer the YR1030 as it requires zeroing every time. The YR1035 retains its zero, but I find I can't trust it without periodic re-zeroing.
These units use the ±1V 1kHz method via 4 wires. They apply a ±1V 1kHz sine wave to the battery and measure its response to compute IR. I use this tool to build small batteries that charge and discharge at 20-25kW and require matched IR.