If I enter Thailand on a triple entry tourist visa when it is almost out of date can I then go to a school and start a Thai language course to get an ED visa?

Lots of visa agents in Thailand will do this for you.

Just google your query and see what comes up.

Here's a start.

You pay for a course of study and get paperwork from school to obtain an ED visa. With class attendance and paperwork you can then extend stay in 90 day blocks from Immigration at cost of 1,900 baht without leaving country again.

One last question, a single entry tourist visa lasts for 60 days so if I apply for a triple entry tourist visa, I wont have to leave to country after 90 days, will I? Can I just wait untill my 180 days are nearly up?

A tourist visa is for 60 days, but can be extended by 30 days for 1,900 baht. After the 60 days (or 90 days if you apply for an extension) you have to leave the country. A triple entry would mean you can make 3 entries of 60 days (or 90 days with extension). You would have to actually leave the country for a new entry after 60/90 days

So even if I have a triple entry tourist visa that I've applied for in my own country (Ireland), I still have to leave every 60 days?

Yes. And you would also have to keep in mind the expiration date of the visa itself, as after the visa expires you can no longer use it to gain entry into Thailand.

If you extend for 30 days as mentioned in post 5 you would only have to leave at the end of 90 days.