There is definitely something in the Irish psyche (for the record, I am Irish myself), which lends itself to a certain negativity and this negativity is evident in a lot of Christy Moore's work. It is undoubtedly the case that life has been hard for the Irish in history but it has been for most of the world. I think if too much emphasis is put on the sad moment or the tragedy, this can lead to distortion. Thankfully, Christy compensates with some really funny tunes at times ("don't forget your shovel").
But, being funny and being joyful belong to different worlds? I think its a question of how one views life and its purpose. It is easier to write a mournful tune than one which is motivated by a belly full of joy as real joy is a rarity. In fairness, though, very few artists/composers manage in their work to evoke joy. I would pick one master of joy and that is Mozart - his Da ponte operas make joy concrete in musical form.
If you like this kind of stuff (and maybe the problem is with me and the ballad form), then best to get his 1991 collection.