ill advised, ag science is relatively easier to understand than the other sciences but there are far more notes involved and i believe you need access to a farm to do project work. If you were desperate you could do it but a good grade would be difficult.
I did chemistry for the first half of fifth year and then changed to geography and I am still not 100% up to speed with the rest of the class.
At the end of the day you have to think is really worth taking on a totally new leaving cert subject with only 6-7 month left before the actual exam. Keep in mind that there is also a project to do as well as learning two years of material. People may tell you that it is easy but you will have the added pressure of learning all this material in such as short space of time along side learning all the material from your other subjects. I would not recommend it.