One of the overarching goals of a DEMO-class device is to demonstrate tritium self-sufficiency in a fusion power plant for the first time. A future power reactor will necessarily require a start-up inventory of tritium, mT start, before commencing fully fledged D-T operations for electricity production. In Europe, it is also presently considered necessary for DEMO to provide a tritium fuel start-up inventory for a subsequent prototype fusion power plant at a certain doubling time, td. At present, there is no model capable of estimating mT start or td for the EU-DEMO, which features a Direct Internal Recycling (DIR) loop in its fuel cycle, and is characterised by low load factors (∼0.2-0.3). This paper introduces a simplified dynamic tritium fuel cycle model capable estimating mT start and td, which has been specifically designed to take into account the effects of low reactor load factors and irregular operation. Results with and without DIR are presented. The fuel cycle design space is explored, and the sensitivity of the performance to variations in key parameters and parameter combinations is analysed. Minimum recommended values of the tritium breeding ratio, load factor, and DIR separation factor are suggested, for the assumptions made herein, based on the response of the fuel cycle performance in the explored design space