Siggery, who stands at 6ft 3in and weighs 16 stone, is a product of the Scarlets academy and has spent the last 18 months playing in the Welsh Premiership.
The move sees Siggery become the second young Scarlet to join the Exiles following their signing of fly-half Steven Shingler.
"Moving to London Irish is a positive step for me. Quite a few of the young lads at the club made a big impact last season and I hope to do the same when I join next season," he said.
Siggery and Shingler played for Wales in Tuesday's record 92-0 defeat to New Zealand Under-20s at the Junior World Championship in Italy.
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The entire All Blacks apparel line has been updated for 2013/14. Check out the New Zealand All Blacks polo.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Gilbert Blitz 8S rugby cleat is a great cleat at a great price of $69.99. Get a new pair of cleats today.
A cool looking all black rugby cleat with the high performance adidas is known for. Get in the Gear!
Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
The All Blacks Performance t-shirt is black with hints of blue from the training jersey. Very Cool.
The New Zealand All Blacks training jersey for 2013/14. Get in the Gear!
The USA Rugby Pro Alternate rugby jersey is perfect for any fan of the Eagles. Get yours to wear during the summer Test matches.
The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.