Legal restrictions mean that direct participants in any lottery need to live in a participating country.
So you need to be in Andorra, Austria, Belgium, France, Ireland, the Isle of Man, Liechtenstein, Luxembourg, Monaco, Portugal, Spain, Switzerland or the UK to participate in Euromillions (and you need to do so through your own country's lottery website).
Similarly, residents of Norway, Sweden, Denmark, Iceland, Finland, Latvia, Lithuania and Estonia can take part in Viking Lotto.
Those in Croatia, the Czech Republic, Denmark, Estonia, Finland, Germany, Hungary, Iceland, Italy, Latvia, Lithuania, the Netherlands, Norway, Slovakia, Slovenia, Spain and Sweden can play the Eurojackpot.
(Note that some of those countries participate in more than one international lottery).
However there's nothing to prevent bookmakers offering odds on the outcomes of national or international lotteries, so gamblers can 'enter' them through bookmakers. (They're not actually buying lottery tickets but placing a bet upon which numbers will be drawn). That's how it's possible to 'play' the Irish lottery in UK betting shops.