The last frost date I believe refers to the last date of air frost.  This is the one that damages plants most.  Ground frost  can happen later in the year but does not damage the plants nearly as much. This is really easy to work out for last year but harder to work out for this year until well into the summer (when it is far too late)  This is where the average last frost date comes in.  Some information below based on statistics I got from Met Éireann.

Of course we know the weather is extremely variable from year to year and it is  really important to note that in the "worst" years the last frost dates are about a month after the average.  The figures below are to give us gardeners a rough idea of the likely last frost date.   Keep in mind that very high or low locations (frost pockets) can have much later last frost dates than others.  Please don't blame me if you move your tender plants out they are damaged by the frost.  Keep an eye on the weather forecast!

 

Average last Spring frost date for the period 1971-2000 at specific weather stations.

I have included a rough idea of the where each place is for those who are geographically challenged (like me).

Station (Roughly located) Average last frost date

Valentia (south west, near the sea) 26 March
Claremorris (West, inland)   28 April
kilkenny (southeast, inland) 29 April
Clones (North, Inland) 26 April

I guess we can say that in the midlands the last frost date is likely to be around the end of April.  Near the sea it can be around a month earlier.  I live in Meath within 2 miles of the sea so my guess is Mid April for my area.