1) Alcohol dilates the arteries in the skin, which results in lower peripheral resistance (the resistance of the arteries to blood flow). In order to maintain normal blood pressure, the heart needs to beat faster and push out larger amount of blood per beat. This makes you aware of your heart beats more.
Relations between alcohol consumption, heart rate, and heart rate variability in men (PubMed Central)
The reason for a positive association between alcohol intake and heart
rate is unclear but possibilities include an increase in sympathetic
activity secondary to vasodilation or increased calcium entering into
2) Dehydration, which causes a drop of blood volume also results in the increased heart rate. Alcohol is a weak diuretic (it slightly stimulates urine excretion), but dehydration after drinking strong alcoholic beverages is more likely caused by insufficient water drinking during alcohol drinking rather than by alcohol itself. Beer (4 vol%), which contains a lot of water actually hydrates you.
Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate (NAP.edu)
...it appears that the effect of ethanol ingestion on increasing
excretion of water appears to be transient and would not result in
appreciable fluid losses over a 24-hour period.