U2 is a slightly stronger album selling artist in Ireland based on a comparision of how well each artist did with the sales of their last albums in Ireland. HTDAAB went 10 times platinum, while West Lifes "face to face" went 8 times platinum in Ireland.
Obviously, U2's 3rd compilation album in 8 years is not going to sell nearly as well as an album of brand new material, and with West Life puting out and album of brand new material with the potential to go 8 times platinum like their last release, it was highly unlikely that a compilation album of songs that most people already have would be able to sell more.
This is a greatest hits album. I don't think one should really be concerned about how well it sells in the first week. Greatest Hits albums are built for longevity, not massive sales in the first weeks of release.