Art is an investment like any other - houses, equities, currencies, vintage cars, wines etc.. It is always prudent to invest in areas where one has a good solid knowledge or can afford to hire an expert. If you don't understand the market or have only rudimentary knowledge of the asset class then it would be unwise to invest, particularly if you can't afford to diversify sufficently as Clubman pointed out.
The Irish Art market is very clubbish, snobbish and inefficent (IMHO). Dublin gallerys take as high as a 50% cut of the sales price in commission. Also remember under new EU law if you resell a work above a fixed amount the orginial artist is entitled to a cut of the proceeds (not of the profit, but of the actual sale price).
Irish Art suffers and will continue to suffer the same affliction as Irish property for the next few years - too much money chasing too few high quality units. There are scarcely a hundred Irish artists who's work is seriously sought after and the scarcity has lead to very high prices for the work of these artists. There "names" command prices perhaps in excess of true values. The very fact that Johnathon Knuttle has carved out such a solid market for himself is testament to this fact.
There are of course thousends of other talented small time artists (such as my mother) who often only paint as a hobby or perhaps as a part time job. There works are frequently displayed in local town art centers. However they are pot luck as far as investment goes, as the majority will never become a "name" and hence will never command much of a resale value.
Althought as I said, Art is an investment like any other, there are several unique aspects to it that make it more complex.
1) unlike property you can't physical improve it's value, i.e. value is completely at the hands of external forces,
2) unlike equities/property there is no useful formula to test value such as p/e or rental yield ratios,
3) art is an objective subject, other asset classes can be viewed as purely invesments whereas Art can conjure significant emotional want in a perspective buyer and hence create a distortion in value.
4) art investment requires more knowlede, research and specialism than almost any other type of major investment i.e. there are tens of thousends of artists in Ireland with varying reputations whereas there are perhaps less than a hundred companies on the Irish stockmarket and all are relatively easy to research (p/e, management, annual results etc.)
5) art has very little physical value or use, only an abstract notional value. This makes true valuation very difficult/impossible. A companies value should be broadly in line with something like "sum of parts"+"growth prospects"+"group premium". A houses value should be something like "cost to build"+"land"+"developers profit"+"inflation since built"+"location premium" (althought this formula no longer seems to hold true in Ireland... but it will return after the slowdown). Arts value is "buyers opinion"+"name reputation" only.
To sum up my point, Art is a very specialist and highly complex market. As you have no real knowledge of it and you are only seeking to invest a relatively small amount (certainly not enough to get proper professional advice), i'd advise you to seek investment elsewhere. You could put the 5k into a First Active Term Account and earn 22.4% gross in 5yrs, or you could start putting the money into an AIB Regular saver account and earn 5.5% gross per annum.
If you really want to invest in art heres a few websites: