Anyway let down serious business. MTB is an awesome sport during in the winter, it's tough and demand a lot of workout on the bleedy hill. For buy second hand bike is a great idea, however I would advice you go to ballinastoe or three rock, they do have man-made MTB trails along with hire MTB bike, so you can test it out for half a day to see if you like it or not. If you love it and want go further than this, then get second hand or brand new bike and get Jamie to teach you on dark art of MTB, Enduro, xc.
I have been into MTB for last 5 years and I love it.
No need to look for one day course, I am sure one of WTC is happy to help you out, he is well knowledge stuff on MTB, i will reach him out and see if he can do it or not. If not there is always plenty of gal or guy doing mtb.
That's good advice from Steve, you should maybe just hire a bike and go for a spin round Ballinastoe for an afternoon and see if you like it, I'd be happy to show you around as I MTB there most Monday evenings after work.