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)

link

In some countries you use all of the titles one holds (e.g. Germany, Switzerland, Netherlands, Belgium etc.)

In plain English it means someone who holds the profession of engineering (can be just bachelors or masters) who has a PhD and who is a Professor (senior ranked, depending on the system can mean a full professor or even an associate prof).

Edit: The tricky bit is what constitutes holding the professing of engineering. In Netherlands/Belgium it is automatic once you have the degree from a technical uni (e.g. Delft or Eindhoven). In some places you need to be a certified engineer to hold it (e.g. Malaysia or Mexico as I understand).

This is true for Germany, but I am not sure about other countries.
The doctoral title you hold, unlike other titles, can be a voluntary addition to the name. It is therefore the only title you can add to your passport.

Based on the situation, you can leave it out though. Most colleagues left it out on their doorbell, some add their field (like the engineering in this case); partially to avoid people with the flu ringing at their door.

Adding your field to your title can help in other situations as well, since it simply conveys what you are doing in three letters or less. People from your, or related, fields can then simply judge what subtopics best to look into, while talking to you at e.g. a conference.

The addition of the Prof. tells people that you are (also) holding a teaching position, or at least where at some point. This shows people even more of what you do. Being yet another bar to pass, some people don't like it being left out, when addressed.

So: trying to tell people more about him, your colleague added eight letters and three dots to his name. Pretty neat, for a CV, right?

EDIT: Changed, based on information given by pieter.