To cite the 'sirt' package in publications use:

Robitzsch, A. (2018). sirt: Supplementary item response theory models. R package version 2.4-20. https://CRAN.R-project.org/package=sirt

Corresponding BibTeX entry:

  @Manual{,
    title = {sirt: Supplementary item response theory models},
    author = {Alexander Robitzsch},
    year = {2018},
    note = {R package version 2.4-20},
    url = {https://CRAN.R-project.org/package=sirt},
  }