What tools are suggested for mortising in large timbers? Is there a preferred modern method or tool that is commonly used?

Depends.

Are you devoted to using hand-tools only?
What species of wood are we talking about?

I'm not totally devoted to using hand tools. I do plan on using chisels, slicks, and hand saws, but I would like some ideas for roughing out the mortises. I tried using a forstner bit and heavy duty drill, but it didn't seem to remove the wood very quickly. I'm only experimenting at this time on pine and hemlock. I've read about people using antique boring tools, but I'm not sure where I would ever locate such a machine.

I'm a beginner in the process of cutting the joints for a barn and am making 2" mortises and tenons in 8"x8" hemlock beams. For hogging out the stopped mortises I am using a cheap 1-7/8" saw-tooth Forstner bit from Woodcraft with a 1/2" Milwaukee 1007 drill. For thru tenons, I am using a 1-3/4 Milwaukee Selfeed bit which I drill halfway thru from each side. This removes material as fast as I am comfortable with. I clean up the mortises with a corner chisel, a 2" framing chisel a 3-1/4" slick, and a swan neck chisel for the stopped mortises.

If you are looking to buy a boring machine, they are available at times. I was looking for one, and emailing with a fellow in Calf. who wanted $240 for his. I've seen them, in excellent shape, sell for $500. I got mine for $50, and a tool dealer had one, same as mine, but worse shape for $85. They are out there but you have to look around. You could try myoldtools.com, in Texas, he has been know to have bits for sale for these, and I don't know if he has boring machines or not. The other choices are a chain mortiser or a chisel mortiser. These are expensive machines but work very well and get the job done quickly. If you get one, and need some bits, the guy who I got my bits from had some more for sale, back then. I'd have to check with him to see if they are still available, if you wanted them. Good Luck. Jim