To cite the 'sirt' package in publications use:

Robitzsch A (2020). sirt: Supplementary Item Response Theory Models. R package version 3.9-4, https://CRAN.R-project.org/package=sirt.

Corresponding BibTeX entry:

  @Manual{sirt_3.9-4,
    title = {sirt: Supplementary Item Response Theory Models},
    author = {Alexander Robitzsch},
    year = {2020},
    note = {R package version 3.9-4},
    url = {https://CRAN.R-project.org/package=sirt},
  }