| Hair | Scotland | Ayrshire | Old Cumnock | 1814 | 3416 | 328 | ||
| Hair | Scotland | Ayrshire | Old Cumnock | 1780 - 1860 | 3416 | 221 | ||
| Halliday | Scotland | Dumfries | 1800 + | 11978 | 1136 | |||
| Halpin | Ireland | Clare | -1812 | 8686 | 657 | |||
| Halpin | Ireland | Clare | pre 1800 | 8049 | 758 | |||
| Hamilton | Scotland | Lanarkshire | Leadhills | -1796 | 5406 | 699 | ||
| Handley | Ireland | Down | Glendown | 1800 s | 2876 | 848 | ||
| Hanrah |
Ireland |
Tyrone |
Clogherny |
1820-1855 |
13164
|
1243 |
||
| Harcus | Scotland | Orkney | Westray | Pre 1855 | 12003 | 1113 | ||
| Hardiman | Ireland | Galway | 1834 - 1835 | 8686 | 634 | |||
| Hardiman | Ireland | Galway | 1851 | 8686 | 635 | |||
| Harris | Ireland | Down | Banbridge | Annaclone | Lisnesligan | 1820 - 1860 | 3818 | 1210 |
| Hayes | Ireland | Down | Killinchy | 1800 - 1899 | 1658 | 779 | ||
| Heenan | Ireland | Galway | 1830 - 1840 | 12602 | 1249 | |||
| Heffernan | Ireland | Limerick | Angelsborough | 1800 + | 10126 | 1101 | ||
| Hegarty | Ireland | Cork | -1792 | 8686 | 653 | |||
| Hegarty | Ireland | Cork | Glanmire | Carrignav | 1826 | 8686 | 655 | |
| Hennessy | Ireland | Tipperary | Clonmel | 1830 - 1840 | 12602 | 1248 | ||
| Hilliard | Scotland | Glasgow | any time | 8958 | 952 | |||
| Hilliard | Ireland | any time | 8958 | 950 | ||||
| Hogan | Ireland | Carlow | Newtown | 1808 - 1840 | 12349 | 1365 | ||
| Horan | Ireland | -1830 | 10713 | 1216 | ||||
| Hughes | Ireland | 1700 - | 7087 | 1306 | ||||
| Hunter | Scotland | Lanark | Lesmanagow | Pre 1828 | 10951 | 1313 | ||
| Hunter | Scotland | Dumfries | Sanquhar | -1759 | 5406 | 867 | ||
| Hurley | Ireland | Clare | Feakie | 1816 | 8686 | 636 | ||
| Ingles | Scotland | East Lothian | Whitekirk | 1750 + | 10126 | 1098 | ||
| Innes | Scotland | Caithness | Latheron | -1870 | 10713 | 1213 | ||
| Ireland | Scotland | Fife | Ceres | 1750 - 1790 | 9165 | 1197 | ||
| Ireland | Scotland | Fife | Cults | 1750 - 1790 | 9165 | 1199 | ||
| Ireland | Scotland | Fife | Kettle | 1750 - 1790 | 9165 | 1201 | ||
| Irvine | Ireland | Tyrone | 1800 - 1860 | 3935 | 881 | |||
| Irvine | Scotland | Dumfriesshire | 1800 - 1899 | 1658 | 1308 | |||
| Jelly | Scotland | Wigtown | Stonykirk | 1800 - 1900 | 12068 | 1297 | ||
| Jenkins | Scotland | Argyll | Dunoon | 1800 | 12660 | 1379 | ||
| Johnson | Scotland | Midlothian | Edinburgh | 1825 + | 10126 | 1329 | ||
| Johnston | Ireland | Queens | Maryborough | Pre 1862 | 10951 | 1316 | ||
| Jolly | Scotland | Wigtown | Stonykirk | 1800 - 1900 | 12068 | 1296 | ||
| Junken | Scotland | Mor | Elgin | 1500 - 1700 | 2968 | 353 | ||
| Kaine | Ireland | Kilkenny | Fassedin | Muckalee | Knockmajor | - 1800 + | 8686 | 761 |
| Kane | Ireland | Kilkenny | Fassedin | Muckalee | Knockmajore | - 1800 + | 8686 | 763 |
| Kane / Cain | Ireland | Kilkenny | Knockmajor | 1820 | 8686 | 814 | ||
| Kearney | Ireland | Dublin | -1815 | 3935 | 372 | |||
| Kelly | Ireland | Kilkenny | Kilkenny | St Patrick | Kilkenny | 1600 - 1800 | 2968 | 339 |
| Kelly | Ireland | Clare | Lisdeen | Kilfearagh | Kilkee | pre 1800 | 8049 | 757 |
| Kelly | Ireland | Clare | -1812 | 8686 | 656 | |||
| Kelly | Ireland | - 1850 - | 12352 | 1183 | ||||
| Kelly | Ireland | Clare | Kilrush | Lisdeen | 1842 | 8686 | 658 | |
| Kelly | Ireland | 1780 - 1850 | 1658 | 192 | ||||
| Kennedy | Scotland | Wigtown | All | 1400 - 1700 | 2968 | 341 | ||
| King | Ireland | Tyrone | Clonfeacl | Grange | 1800 + | 3416 | 319 | |
| King | Ireland | Dublin | 1797 - 1829 | 12349 | 1366 | |||
| King | Ireland | Tyrone | Coolkill | Clonfeacl | Moy | 1790 - 1870 | 3416 | 212 |
| King | Ireland | Tyrone | Grange | Clonfeacl | 1790 - 1890 | 3416 | 213 | |
| Lane | Ireland | Queens | Marybirough | Pre 1862 | 10951 | 1317 | ||
| Lang | Scotland | Lanark | Leadhills | -1832 | 5406 | 869 | ||
| Lang | Scotland | Lanarkshire | Crawford | 1864 + | 5406 | 715 | ||
| Lang | Scotland | Lanarkshire | Govan | 1860 + | 5406 | 716 | ||
| Lang | Scotland | Ayrshire | Kilmarnoc | pre 1859 | 5406 | 697 | ||
| Lapsley | Scotland | Kirkcudbrightshire | Minnigaff | Blackcraig | - 1780 - | 12352 | 1181 | |
| Lawler | Ireland | Wicklow | Dunlanin | 1790 - 1870 | 5556 | 1264 | ||
| Lawson | Scotland | 1803 - 1820 | 6503 | 875 | ||||
| Lawson | Scotland | Banff | Inverkeithny | 1842 - 1850 | 8686 | 776 | ||
| Lee | Ireland | Tipperary | 1800 - 1820 | 12660 | 1372 | |||
| Lees | Scotland | Ayrshire | Sorn | Catrine | 1814 | 3416 | 329 | |
| Lees | Scotland | Ayrshire | Old Cumnock | 1780 - 1850 | 3416 | 219 | ||
| Lennon | Ireland | Derry | -1824 | 5406 | 865 | |||
| Lennox | Scotland | Fife | 1750 + | 11978 | 1217 | |||
| Lennox | Scotland | Perthshire | Kilmadock | C 1738 | 3487 | 725 | ||
| Leonard | Ireland | Tyrone | 1820 - 1840 | 12602 | 1252 | |||
| Lettice | Ireland | Meath / Kildare | 1800 - 1899 | 1658 | 193 | |||
| Liddle | Scotland | Orkney | Eday / Stronsay | Pre 1855 | 12003 | 1114 | ||
| Lindsay | Scotland | Fife | - 1820 + | 4705 | 1352 | |||
| Lindsay | Ireland | Tyrone | Kilskeery | Trillick | -1850 | 3935 | 364 | |
| Lindsay | Ireland | Fermanagh | Enniskill | Irvinesto | -1850 | 3935 | 365 | |
| Lockhart | Scotland | Ayrshire | Dundonald | 1780 - 1860 | 10976 | 1283 | ||
| Lockhart | Scotland | Ayrshire | Dailly | 1780 - 1860 | 10976 | 1282 | ||
| Low | Scotland | Fife | Leslie | 1800 - 1850 | 3818 | 1207 | ||
| Lynch | Ireland | Sligo | Sligo | 1823 | 4335 | 6 |