Is it legal to use a metal detector on Irish and Northern Irish beaches?

To expand on James answer, it is not just a carry on of UK law that regulates Metal Detecting in the Republic. Basically, metal detecting is HIGHLY regulated in the south and is basically illegal if it deemed to be in search of anything archealogical.

The National Monuments (Amendment) Act

1987

(Section 2) states:

Subject to the provisions of this section a person shall not:

1a: Use or be in possession of a detection device in, or at the site of, a monument

of which the Commissioners or a local authority are the owners or guardians or in

respect of which a preservation order is in force or which stands registered in the

Register or

2a. in an archaeological area that stands registered in the Register or

3a. in a Registered area

OR

b: Use, at a place other than a place specified in paragraph a of this subsection, a detection device for the purpose of searching for archaeological objects or

c: Promote, whether by advertising or otherwise, the sale or use of detection

devices for the purpose of searching for archaeological objects.

Note: `Archaeological area' is defined as ` an area which the Commissioners

consider to be of archaeological importance but does not include the area of a

historical monument standing entered in the Register'.

Section 40 states that `Where in a prosecution for an offence under this section it

is proved that a detection device was used, it shall be presumed until the contrary

is proved that the device was being used for the purpose of searching for

archaeological objects'.

Yes it is legal but due to the law of "Treasure Trove", if you find anything significant you have to hand it over to the authorities.

http://en.wikipedia.org/wiki/Treasure_trove

(That Wikipedia article refers to the UK, but the law was carried over to the Irish Republic when it gained independence.)

You might be given a reward for anything you find though, and are entitled to recognition for your find (i.e. if it is put on display in a museum you will be listed as the finder).

Also, if it is found that it is not historically or culturally significant, whatever you found will probably be given to you to keep (provided they cannot find its rightful owner).

Who would know anyway?

It's not like there'll be anyone watching you walking up and down the beach.