What Mark says about says Data Centres omits to include suppliers, outsourced support staff, building maintenance and others I can't immediately think of. The numbers employed in total may not be huge but there are other contributions including construction employment, support for renewable energy and conmercial rates. More importantly other employment and activity may follow in the area with obvious societal and region benefits.
As for the WRC, the case for completion may not be overwhelming but despite motorways Galway is a heavily congested city and both it and it's hinterland would benefit from improved regional rail links. The other key advantage of re-instating the Athenry to Galway line is that it would allow diversion of freight traffic destined for Waterford away from the Kildare bottleneck.