Try here, that said, the first was last updated in 2009 and I,m fairly sure the Kings Arms session in Holyhead is no more. The second is the nearest to Abersoch I can find. I'm also not sure how current it is.
Weekly & Monthly Folk Music Clubs & Venues in North Wales
The Golden Fleece Inn, Tremadog
If you are really stuck try emailing the Coastguard Sector Manager in Tremadog and asking him. His name is Derek Roberts and we used to do the Tuesday Irish session in Holyhead together when I was at the CG station there. He also plays melodion and guitar so may even be at the session himself if it is still running.
His email is firstname.lastname@example.org
Tell him Chris (ex Traedmoch) sent you.
Hope this helps.