DUBLIN, April 29 (Reuters) - Ireland's annual inflation rate slowed to 1.6% in April from 1.7% in March and the core rate excluding energy and unprocessed food prices fell to 2.6% from 2.9%, a flash estimate of the Harmonised Index of Consumer Prices (HICP) showed.

Separate flash data on Monday suggested first-quarter gross domestic product (GDP) increased 1.1% quarter-on-quarter. Irish GDP estimates can be sharply revised and last month's final figure for a contraction in Q4 was amended to 3.4% from 0.7%.

The government has long cautioned against using GDP to accurately measure economic growth as it is routinely impacted by multinational activity. Its preferred measure, modified domestic demand, is not included in the flash estimates. (Reporting by Padraic Halpin, Editing by Louise Heavens)