I have been told that it is possible to get a 3 month Work Visa for Tanzania at the airport at a cost of USD150. However, the Tanzanian H C web site says that you need a visa before you travel. I have an Irish passport - can I get a Work Visa at the airport?

no, it is not possible. trust me. you will get a tourist visa for 3 months (no work, no salary), then you can apply for work visa (1 year, 2 years, depending on whether you are a researcher or earning money in the country). go to the embassy, but out of personal experience, they are not really helpful. do not try to do a work visa on your own - it is gonna be a nightmare. you need locals or organisations in tanzania to take care of it.
I'd get your tourist visa on arrival (free for us Irish!) and then follow the advice above & get it done by 'those in the know'
Kira

To enter, you will get a visa. USD50 and it is valid for 3 months. as malaika03 says, it is for tourism.
To work in the country, you need a residence permit. You'll apply for it in Immigration in the country. There are different permits (and requirements) depending on the type of work/organisation. TZ Residence permits. You need certain documents from your employer (see the website)r, in that sense you can't sort it out alone, however, you can handle the process by yourself if you are a bit assertive, cordial and patient.

Thanks for all the info. As I will be working on a grant aid project attached to Tanroads I guess they will assist me in getting a Residence/Work Permit. I start work on the project the day I arrive in Tanzania - will it be difficult to transfer from a Tourist Visa to a Residence Permit? I don't have the option of going to a Tanzanian Embassy before hand as I am working in the south of India!
Thanks again

You don't need to deal with the Embassy, if you need a residence permit. You get the visa at the airport; and when you are in the country, obtain all the necessary docs from your employer (and yourself) and submit your application in the Immigration. However, usually companies have their own guys to deal with issues like this, so that you don't need to take care of the process by yourself (and lose your time). In my opinion it is a normal procedure, a bit cumbersome and takes some time. My permit took about 6 weeks, if i recall right.
However, it came to my mind, are you sure a business visa would not cover your needs? Are you going to work as a volunteer or as a short term consultant (TA) for eg a project? When i have worked as a short term consultant in other African countries, i used a business visa, which is obtainable in the Embassy. Now, i don't know if that applies to Tanzania as well. A volunteer applies for a Class C residence permit.
Kira, are you sure you Irish still get the visa for free? Last weel I got talking to an Irish girl at the airport in Dar, and she had to pay $100 (!) for a single entry tourist visa. She said there was a sign up at the visa counter stating that as of October 20 (or so) all others pay $50, while Irish passport holder pay $100...