The constant interruptions were a source of great irritation to the speaker.

irritation 是名词，由动词 irritate（激怒、使烦躁）加上名词后缀 -ion 构成，表示"恼怒、烦躁"的状态或感觉。后缀 -ion 通常用来将动词转化为表示状态或结果的名词。