What does “Prof. Dr. ir.” mean?
Ir. means a masters degree in Engineering. 

source : Belgian Education :)

I added a "reference". This Prof. Dr. Ir. received a M.Sc. degree in electrical engineering and the Ph.D. degree in mechanical engineering from the Katholieke Universiteit Leuven (K.U.Leuven), Belgium

Also, from wikipedia:
Belgium

In Belgium, there are two types of engineering degrees:


"Burgerlijk Ingenieur" /"Ingénieur civil" or "Master of Science in Engineering" (abbrev. "ir.") - 5 years study (3 BSc. + 2 MSc.)
"Bio-ingenieur"/ "Bioingénieur" or "Master of Science in Bioscience Engineering" (abbrev. "ir") - 5 years study (3 BSc. + 2 MSc.)
"Industrieel Ingenieur" or "Master of Science in Industrial Sciences" (abbrev. "Ing.") - 4 years study (3 BSc. + 1 MSc.)(3 BSc. + 2 MSc. in 2013-future) (for the Flemish Region)
"Ingénieur industriel" or "Master of Science in Industrial Sciences" (abbrev. "Ing.") - 5 years study (3 BSc. + 2 MSc.) (for the Walloon Region)


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