not sure if you've heard them already but kyuss (Blues for the red sun seems to be the one everyone loves, although i'm more of a ...circus leaves town fan), Sleep (dopesmoker for one looooooong monster of a track or holy mountain for more conventional album structure but still heavy stuff), high on fire (ex-sleep), om (ex-sleep), others include sunn o))), earth, bongzilla, clutch, mastodon, pelican
from the uk:gonga (pretty much a kyuss rip off but still good) hawkwind (from the 70s but still heavy, check out space ritual) sabbath (goes without saying)
less metal-y but still heavy and noisy: jesus lizard, melvins, burning brides, tomahawk, icarus line
there's some good heavy stuff in ireland too, you just have to know where to look,
http://www.myspace.com/saturnaliaband
http://www.badboat.net/
http://www.hexxed.co.uk/
http://www.traceramc.co.uk/index.php (the closest irish band to sonic youth that i know of)