The domestic side of diplomacy, Up in the Park, by Elizabeth Shannon. New York: Atheneum. 358 pp. $17.95.

Elizabeth Shannon was the kind of ambassador's wife who could man a mop when the roof leaked in the morning, carry the message of American feminism to a group of Irishwomen in the afternoon, and entertain diplomats at dinner in the evening.

With originality and charm, she tells the story of her years in Ireland (her husband was US ambassador to Ireland from 1971 to '81) in the form of a diary, which she calls her ''sympathetic and forgiving friend to whom I pour out each day's news.''

In one typical entry, she confides: ''...I fell in love with Ireland's beauty all over again, as I do about twice a week.'' And then, ''I've run out of small talk and I'm overflowing with coffee and cakes.''