This paper presents two distinct but related empirical analyses of the Irish Day-Ahead electricity market, which underwent a structural change during the study horizon due to a Brexit-related shift in market coupling. First, we examine the activity of speculators (i.e. financial traders), measuring their overall market share in terms of order volumes and executed trades. We also assess the proportion of trading periods in which speculators are marginal, that is when their bid or offer prices equal the market-clearing price. Despite their relatively small share of total market activity, we find that speculators are marginal in a significant percentage of trading periods. We also identify a clear shift in their behavioural patterns following the structural market change. The second analysis introduces an intuitive method to quantify price inertia, the extent to which market prices resist small changes in demand or supply. While conceptually similar to liquidity or order book depth in continuous trading environments, this approach is applied within the context of a Day-Ahead auction market. We observe that price inertia levels also change following the structural shift. The Irish market offers a representative and transparent setting for short-term European electricity markets, allowing for clear identification of participant types. A clearer understanding of both aspects is essential for regulators evaluating market design as well as for market participants managing risk in Day-Ahead markets.