If you have it in a proper 50l barrel (keg?) it needs to be hooked up to a CO_2 bottle on the tap
That is absolute rubbish!
CAMRA has this definition: Real ale is a beer brewed from traditional ingredients (malted barley, hops water and yeast), matured by secondary fermentation in the container from which it is dispensed, and served without the use of extraneous carbon dioxide. (Emphasis mine)
Beer that is secondary fermented (which includes traditional lagers) in the barrel continues to generate enough CO2 to maintain the beer in good condition throughout the serving life of the cask (which should normally be about three to five days). The beer can be drawn direct from the barrel (gravity), via a traditional handpump/beer engine, or by electric pump (frowned upon by purists :) )
You do not (and never have) needed extraneous CO2 to dispense proper beer; brewers prior to the widescale bottling of CO2 managed to get on pretty well without it!
The lines in a traditional pub or bar were short enough to allow the drawing of beer by hand without the need for extraneous gas (yes it needed some muscle power but not much - I've worked in numerous pubs with unassisted handpumps and I'm no Hercules!). It is only modern pubs built since CO2 dispense became normal (those massive 60s estate pubs, shop conversions of the Wetherspoons type) that some pubcos have resorted to CO2 assist to dispense real ale.