To cite irtrees in publications use:

Paul De Boeck, Ivailo Partchev (2012). IRTrees: Tree-Based Item Response Models of the GLMM Family. Journal of Statistical Software - Code Snippets, 48(1), 1-28. URL http://www.jstatsoft.org/v48/c01/.

Corresponding BibTeX entry:

  @Article{,
    title = {IRTrees: Tree-Based Item Response Models of the GLMM
      Family},
    author = {Paul {De Boeck} and Ivailo Partchev},
    journal = {Journal of Statistical Software -- Code Snippets},
    year = {2012},
    volume = {48},
    number = {1},
    pages = {1--28},
    url = {http://www.jstatsoft.org/v48/c01/},
  }