Where can I find light-weight glass blocks or a glass block substitute
for building glass block or glass brick walls in Ireland?

Yeh Roadstone, the Blocks are cheap, its the special cement you need
thats expensive, Blocks vary in size, my advice look at size of blocks
first, then figure the size of your frame, unless you have a GLASS
Cutter your up Sh1t creek without a paddle.

I wish somebody had told me that before I started. 
Great visual now, but god it took me forever to get it right.

PolarLight based in Newry, County Down (+44 (0)28 3083 3888
www.polarlight.ie) sell acrylic block panels.
These look exactly like glass blocks or glass bricks, but are only a
fraction of the weight. You don't have to build them yourself. They
make the panels, windows, or walls in their factory and deliver them
to you in one piece. They then just have to be screwed into place. We
have two - one in the bathroom, and one as part of an external wall.
There is really smooth white silicon finish between the blocks, which
looks much better than the grout normally between glass blocks.

They also sell a system for building glass blocks (or their acrylic
blocks) yourself. It all locks together like lego. They have an
internal lighting option for these too. But if you are looking for
lightweight, I would go for their finished panels. I priced building
glass blocks with the steel rods and mortar and the acrylic block
panels worked out about one third cheaper.

Thanks Denis,

Had a look at www.polarlight.co.uk
and it seems exactly what I'm looking for.

Having the panels already made and shipped to me in one piece seems a
lot easier than building glass blocks with steel rods and mortar. The
problem I had was that I needed to strengthen the floor to take the
weight of the glass block wall. These PolarLight acrylic block panels
appear to be very light.