This is really cool; Gaelic acapella psalm singing. Be sure to listen to a few of the audio clips. It sounds much less like traditional "lining-out" psalm-singing and much more like the call and response of gospel music; there's an article on the site that supports that possible historic link.
I mentioned a few posts back an argument for singing only the Psalms, vs. singing the Psalms as well as other parts of Scripture. The main thought there is that from God's action of closing of the Psalter - an event we know did happen even thought we don't know exactly when - we ought to draw the conclusion that this is what God wants us to sing. (By "closing", I'm referring to that point in which the Psalter was, by God's standard, completed and not open to any additions or subtractions.) The picture is God handing to His people a book of songs we know He desires us to sing in worship; that very action seems to carry the weight of exclusive command. It should be pointed out that the lack of divine command to sing other songs is a much stronger argument against singing other parts of Scripture; all the while, we must bear in mind the perfection of the Psalter as our only-needed hymnal.
While there were some Old Testament non-Psalm songs sung in celebration and worship (Exodus 15), these were before the closing of the Psalter. Once the Psalter is closed, we have no examples of other songs sung in corporate worship. The most obvious counter arguments would be Mary's magnificat (Luke 1) and the several songs scattered through John's Revelation. In response to the first, I would still point out that we have no command to use this as a song in worship (although each and every statement of the magnificat has wonderful cross references in the Psalms). In response to the songs of Revelation, I would again point out that the material in those songs is not unfamiliar to the singer of Psalms and that the visions in Revelation do not directly correspond to the called, corporate worship of God's people in these last days. This is only to say that Revelation is not designed to be our theology-of-worship manual (it's purpose is much greater).
I'm afraid this may sound like a denigration of other parts of Scripture; certainly that is not intended. My concern is not to put down other Scripture, but simply to be faithful to what God has called us to do in worship. If He has called us to sing the magnificat, then by all means let us sing. If not, then let's believe that what God has given us is (1) what He wants and (2) far more than good enough for us.