Final answer:
Surface agraphia is the condition that hinders the spelling of irregular words but not regular words due to issues accessing orthographic word representations.
Step-by-step explanation:
The form of central agraphia that involves impairment in spelling irregular words such as 'island' but not regular words like 'dog' is known as Surface agraphia. Surface agraphia is characterized by difficulty in accessing the orthographic representations of words, which leads to the incorrect spelling of words that do not follow regular phonetic rules. However, the ability to spell phonetically regular words, which can be spelled as they sound, remains intact.