Welsh emphasis is penultimate so bri-ALL-en...now...LL in Welsh has a very foreign and hard-to-master sound. Just in case you want to make the effort, say 'LLLLLLLLL' and feel how your tongue moves and where it goes. Now put the tip of your tongue behind your upper front teeth like you're going to say 'L' but instead of using your vocal chords, just blow (aspirate if you want to be fancy ;o). Now, outside of Wales, that's a very nice gesture but wholly unnecessary. Bri- (as in 'fit') AL (as in 'palm') and len (as in 'get').
The plural is Briallu, never used as a name but perhaps helpful in sorting out the pronunciation.