First off, if you're going to criticise something for lacking scientific rigour, then you'd better have something other than a glance out the window of the car to back it up. Swallows were recorded in Cork at the March, just because you didn't see them until yesterday means nothing.As you know, 2009, 2010, and 2011 were pretty cold winters in Ireland. Spring temperatures were also on the very low side.
This year was one of the warmest Springs on record in Ireland yet it had no bearing on the arrival time of the swallows into Ireland.
The 1st swallow I saw was yesterday, 16th of April and in the years previous with an exceptionally cold spring, was 13th & 14th of April.
Second, the harshness of the inter in Ireland isn't a critical factor. They don't look up Met.ie from Africa and barrell on up. The weather all along their route is what matters. If wintery weather persists late into spring here they will stay south of here for longer. If they get mild weather all the way up, then they'll move north at a quicker rate.