| # | Name | Origin | Meaning | Gender | |
|---|---|---|---|---|---|
| 661 | Roy | Celtic | Red haired. | M | |
| 662 | Sativola | Celtic | Name of a saint. | M | |
| 663 | Sawyer | Celtic | Cuts timber. | M | |
| 664 | SciIti | Celtic | Legendary messenger of Arthur. | M | |
| 665 | Selma | Celtic | Comely. | F | |
| 666 | Setanta | Celtic | Mythical son of Sualtam. | M | |
| 667 | Sheridan | Celtic | Untamed. | M | |
| 668 | Sloan | Celtic | Fighter. | M | |
| 669 | Sloane | Celtic | Fighter. | M | |
| 670 | Sugn | Celtic | Mythical son of Sugnedudd. | M | |
| 671 | Tadhg | Celtic | Mythical a prince of Munster. | M | |
| 672 | Taliesin | Celtic | Bard. | M | |
| 673 | Tanguy | Celtic | Fighter. | M | |
| 674 | Teaghue | Celtic | Bard. | M | |
| 675 | Teague | Celtic | Bard. | M | |
| 676 | Teithi | Celtic | Mythical son of Gwynham. | M | |
| 677 | Teyrnon | Celtic | Regal. | M | |
| 678 | Tiernan | Celtic | Regal. | M | |
| 679 | Tiernay | Celtic | Regal. | M | |
| 680 | Torrey | Celtic | Lives by the tower. | M | |