I visited this site that mattycat added to the Ireland Links List Topic:

Irish Ancestors - and discovered that you can do a surname search at this page........ http://www.ireland.com/ancestor/surname/

So for interest - did a search for Smith.
Got these results.......

[nb: on the actual results page at that site, the county names are clickable - to go to a breakdown of the number of households by parish (paying).]

The table below shows the number of smith households in each county in the Primary Valuation property survey of 1848-64.
Antrim 273 Armagh 189
Belfast city 176 Carlow 35
Cavan 1074 Clare 30
Cork 150 Cork city 39
Derry 213 Donegal 91
Down 635 Dublin 212
Dublin city 159 Fermanagh 117
Galway 123 Kerry 25
Kildare 82 Kilkenny 50
Laois 45 Leitrim 73
Limerick 41 Limerick city 6
Longford 102 Louth 156
Mayo 79 Meath 578
Monaghan 273 Offaly 88
Roscommon 44 Sligo 82
Tipperary 142 Tyrone 214
Waterford 42 Westmeath 176
Wexford 62 Wicklow 105

SURNAME DICTIONARY/ SLOINNTE NA h-EIREANN
Smith Very numerous: all areas: particularly Leath Chuinn (northern half). Ir. Mac Gabhann, which is linked to Cavan. See Mac Gowan and Smyth.
Smythe Very numerous: all areas, especially Ulster and Midlands. Ir. Mac Gabhann, Mac an Ghabhann (son of the smith). The Irish name was widespread and generally changed to Smith/Smyth. Branches in Clare and Tipperary were traditional historians. Many are also of Scottish or English origin in Ulster. See also Mac Gowan. SGG & IF.